Augusta Ada King, Countess of Lovelace (née Byron; 10 December 1815 – 27 November 1852), also known as Ada Lovelace, was an English mathematician and writer chiefly known for her work on Charles Babbage's proposed mechanical general-purpose computer, the Analytical Engine. She was the first to recognise that the machine had applications beyond pure calculation. Ada Lovelace is sometimes considered to be the first computer programmer.

Ada Lovelace was the only legitimate child of poet Lord Byron and reformer Anne Isabella Milbanke. All her half-siblings, Lord Byron's other children, were born out of wedlock to other women. Lord Byron separated from his wife a month after Ada was born and left England forever. He died in Greece whilst fighting in the Greek War of Independence, when she was eight. Lady Byron was anxious about her daughter's upbringing and promoted Lovelace's interest in mathematics and logic in an effort to prevent her from developing her father's perceived insanity. Despite this, Lovelace remained interested in her father, naming one son Byron and the other, for her father's middle name, Gordon. Upon her death, she was buried next to her father at her request. Although often ill in her childhood, Lovelace pursued her studies assiduously.

She married William King in 1835. King was already a Baron, and was created Viscount Ockham and 1st Earl of Lovelace in 1838. The name Lovelace was chosen because Ada was descended from the extinct Baron Lovelaces. The title given to her husband thus made Ada the Countess of Lovelace.

Lovelace's educational and social exploits brought her into contact with scientists such as Andrew Crosse, Charles Babbage, Sir David Brewster, Charles Wheatstone and Michael Faraday, and the author Charles Dickens, contacts which she used to further her education. Lovelace described her approach as "poetical science" and herself as an "Analyst (& Metaphysician)".

When she was eighteen, Lovelace's mathematical talents led her to a long working relationship and friendship with fellow British mathematician Charles Babbage. She was in particular interested in Babbage's work on the Analytical Engine. Lovelace first met him on 5 June 1833, when she and her mother attended one of Charles Babbage's Saturday night soirées with their mutual friend, and Lovelace's private tutor, Mary Somerville.

Though Babbage's Analytical Engine was never constructed and exercised no influence on the later invention of electronic computers, it has been recognised in retrospect as a Turing-complete general-purpose computer which anticipated the essential features of a modern electronic computer; Babbage is therefore known as the "father of computers," and Lovelace is credited with several computing "firsts" for her collaboration with him.

Between 1842 and 1843, Lovelace translated an article by the military engineer Luigi Menabrea (later Prime Minister of Italy) about the Analytical Engine, supplementing it with seven long explanatory notes. These notes described a method of using the machine to calculate Bernoulli numbers which is often called the first published computer program.

She also developed a vision of the capability of computers to go beyond mere calculating or number-crunching, while many others, including Babbage himself, focused only on those capabilities. Lovelace was the first to point out the possibility of encoding information besides mere arithmetical figures, such as music, and manipulating it with such a machine. Her mindset of "poetical science" led her to ask questions about the Analytical Engine (as shown in her notes), examining how individuals and society relate to technology as a collaborative tool.