Augusta Ada King, Countess of Lovelace (10 December 1815 – 27 November 1852), born Augusta Ada Byron, was an English writer chiefly known for her work on Charles Babbage's early mechanical general purpose computer, the analytical engine. Her notes on the engine include what is recognised as the first algorithm intended to be processed by a machine; as such she is regarded as the world's first computer programmer.

She was the only legitimate child of the poet Lord Byron (with Anne Isabella Milbanke), but had no relationship with her father, who died when she was nine. As a young adult she took an interest in mathematics, and in particular Babbage's work on the analytical engine. Between 1842 and 1843 she translated an article by Italian mathematician Luigi Menabrea on the engine, which she supplemented with a set of notes of her own. These notes contain what is considered the first computer program — that is, an algorithm encoded for processing by a machine. Though Babbage's engine was never built, Lovelace's notes are important in the early history of computers. She also foresaw the capability of computers to go beyond mere calculating or number crunching while others, including Babbage himself, focused only on these capabilities.

Ada Augusta Byron, born on 10 December 1815, was the only child of the poet George Gordon, 6th Lord Byron, and his wife, Anne Isabella "Annabella" Milbanke, Baroness Wentworth. Byron, and many of those who knew Byron, expected that the baby would be "the glorious boy", and there was some disappointment at the contrary news. She was named after Byron's half-sister, Augusta Leigh, and was called "Ada" by Byron himself. On 16 January 1816, Annabella, at Byron's behest, left for her parents' home at Kirkby Mallory taking one month old Lovelace with her. Although English law gave fathers full custody of their children in cases of separation, Byron made no attempt to claim his parental rights. On 21 April, Byron signed the Deed of Separation, although very reluctantly, and left England for good a few days later. Byron did not have a relationship with his daughter and he died in 1824 when she was nine; her mother was the only significant parental figure in her life. Her mother, Annabella, became Baroness Wentworth in her own right in 1856, being then the sole remaining representative of the Wentworth Viscounts. Lovelace was often ill, dating from her early childhood. At the age of eight she experienced headaches that obscured her vision. In June 1829, she was paralysed after a bout of the measles. She was subjected to continuous bed rest for nearly a year, which may have extended her period of disability. By 1831 she was able to walk with crutches.

Throughout her illnesses, Lovelace continued her education. Her mother's obsession with rooting out any of the insanity of which she accused Lord Byron was one of the reasons that Lovelace was taught mathematics from an early age. Lovelace was privately schooled in mathematics and science by William Frend, William King and Mary Somerville. One of her later tutors was the noted mathematician and logician Augustus De Morgan. From 1832, when she was seventeen, her remarkable mathematical abilities began to emerge, and her interest in mathematics dominated her life even after her marriage. In a letter to Lovelace's mother, De Morgan suggested that Lovelace's skill in mathematics could lead her to become "an original mathematical investigator, perhaps of first-rate eminence".

Lovelace never met her younger half-sister, Allegra Byron, daughter of Lord Byron and Claire Clairmont, who died in 1822 at the age of five. She did, however, have some contact with Elizabeth Medora Leigh, the daughter of Byron's half-sister Augusta Leigh. Augusta Leigh purposely avoided Lovelace as much as possible when she was introduced at Court. Lovelace knew Mary Somerville, noted researcher and scientific author of the 19th century, who introduced her to Charles Babbage on 5 June 1833. Other acquaintances were Sir David Brewster, Charles Wheatstone, Charles Dickens and Michael Faraday.

By 1834, Lovelace was a regular at Court and started attending various events. She danced often and was able to charm many people and was described by most people as being dainty. However, John Hobhouse, Lord Byron's friend, was the exception and he described her as "a large, coarse skinned young woman but with something of my friend's features, particularly the mouth". This description followed their meeting on 24 February 1834 in which Lovelace made it clear to Hobhouse that she did not like him, probably due to the influence of her mother, which led her to dislike all of her father's friends. This first impression was not to last, and they later became friends.

On 8 July 1835 she married William King, 8th Baron King, later 1st Earl of Lovelace in 1838. Her full title for most of her married life was "The Right Honourable the Countess of Lovelace". Their residence was a large estate at Ockham Park, in Ockham, Surrey, along with another estate and a home in London. They had three children; Byron born 12 May 1836, Anne Isabella (called Annabella, later Lady Anne Blunt) born 22 September 1837 and Ralph Gordon born 2 July 1839. Immediately after the birth of Annabella, Lovelace experienced "a tedious and suffering illness, which took months to cure".

In 1841, Lovelace and Medora Leigh (daughter of Lord Byron's half-sister Augusta Leigh) were told by Lovelace's mother that Byron, her father, was also Medora's father. On 27 February 1841, Lovelace wrote to her mother: "I am not in the least astonished. In fact you merely confirm what I have for years and years felt scarcely a doubt about, but should have considered it most improper in me to hint to you that I in any way suspected". Lovelace did not blame the incestuous relationship on Byron, but instead blamed Augusta Leigh: "I fear she is more inherently wicked than he ever was". This did not prevent Lovelace's mother from attempting to destroy her daughter's image of her father, but instead drove her to attack Byron's image with greater intensity.

Ada Lovelace met and corresponded with Charles Babbage on many occasions, including socially and in relation to Babbage's Difference Engine and Analytical Engine. Babbage was impressed by Lovelace's intellect and writing skills. He called her "The Enchantress of Numbers". In 1843 he wrote of her:

Forget this world and all its troubles and if

possible its multitudinous Charlatans — every thing
in short but the Enchantress of Numbers.

During a nine month period in 1842 – 43, Lovelace translated Italian mathematician Luigi Menabrea's memoir on Babbage's newest proposed machine, the Analytical Engine. With the article, she appended a set of notes. The notes are longer than the memoir itself and include (Section G), in complete detail, a method for calculating a sequence of Bernoulli numbers with the Engine, which would have run correctly had the Analytical Engine ever been built. Based on this work, Lovelace is now widely credited with being the first computer programmer and her method is recognised as the world's first computer program. However, some biographers debate the extent of her original contributions. Dorothy Stein, author of Ada: A Life and a Legacy, contends that the programs were mostly written by Babbage himself. Babbage wrote the following on the subject, in his Passages from the Life of a Philosopher (1846):

I then suggested that she add some notes to Menabrea's memoir, an idea which was immediately adopted. We discussed together the various illustrations that might be introduced: I suggested several but the selection was entirely her own. So also was the algebraic working out of the different problems, except, indeed, that relating to the numbers of Bernoulli, which I had offered to do to save Lady Lovelace the trouble. This she sent back to me for an amendment, having detected a grave mistake which I had made in the process.

The level of impact of Lovelace on Babbage's engines is difficult to resolve due to Babbage's tendency not to acknowledge (either orally or in writing) the influence of other people in his work. However, Lovelace was certainly one of the few people who fully understood Babbage's ideas and created a complex program for the Analytical Engine; indeed there are numerous clues that she might also have suggested the usage of punched cards for Babbage's second machine since her notes in Menabrea's memoir suggest she deeply understood the Jacquard loom as well as the Analytical Engine. Her prose also acknowledged some possibilities of the machine that Babbage never published, such as speculation that "the engine might compose elaborate and scientific pieces of music of any degree of complexity or extent".

Lovelace died at the age of thirty-six, on 27 November 1852, from uterine cancer and bloodletting by her physicians. She was buried, at her request, next to her father at the Church of St. Mary Magdalene in Hucknall, Nottingham.

In 1842 Charles Babbage was invited to give a seminar at the University of Turin about his analytical engine. Luigi Menabrea, a young Italian engineer, and future prime minister of Italy, wrote up Babbage's lecture in French, and this transcript was subsequently published in the Bibliothèque Universelle de Genève in October 1842. Babbage asked the Countess of Lovelace to translate Menabrea's paper into English, subsequently requesting that she augment the notes she had added to the translation. Lady Lovelace spent most of a year doing this. These notes, which are more extensive than Menabrea's paper, were then published in The Ladies' Diary and Taylor's Scientific Memoirs under the initialism "A.A.L.". In 1953, over one hundred years after her death, Lady Lovelace's notes on Babbage's Analytical Engine were republished. The engine has now been recognised as an early model for a computer and Lady Lovelace's notes as a description of a computer and software. Her notes were labelled alphabetically from A to G. In note G, the Countess describes an algorithm for the analytical engine to compute Bernoulli numbers. It is generally considered the first algorithm ever specifically tailored for implementation on a computer, and for this reason she is considered by many to be the first computer programmer.

The computer language Ada, created on behalf of the United States Department of Defense, was named after Lovelace. The reference manual for the language was approved on 10 December 1980, and the Department of Defense Military Standard for the language, "MIL-STD-1815", was given the number of the year of her birth. Since 1998, the British Computer Society has awarded a medal in her name and in 2008 initiated an annual competition for women students of computer science.

In popular media, Lady Lovelace has been portrayed in the film Conceiving Ada and the book The Difference Engine by William Gibson and Bruce Sterling.

24 March has been commemorated by some as Ada Lovelace Day, a day to celebrate the achievements of women in technology and science.