Depending upon how one traverses, with rising panic, the first vowel of this word, this can mean - since we are here to talk of universal fact, not your sweetheart or my secretary - either 1 a programming language or 2 the best novel yet written, by Vladimir Nabokov.
Others are better qualified to talk of 1, though those keen on any extractable piece of human interest might wish to know that Ada was named after, though not invented by, Lady Ada Lovelace, a clumsy and glum little grub and the daughter of Lord Byron. The larval Ada was taught mathematics only because she was thought too hideous to marry; she went on to invent programming, in connection with Babbage, and, while she was at it, metamorphosed into a famous beauty. Something similar happens in the brain of the user introduced to the language. At first he blinks at its ugliness and verbosity, remembering that it was produced by an American government department, no doubt during a Democrat administration. If forced to continue, he comes to find that what he has written in Ada actually works. The language comes to seem not prolix and niggling but stately and correct. The user tosses aside his volumes of Nozick and K&R and blesses the government machine.
On to 2. This is trickier, so I will not say too much about Ada or Ardor: A Family Chronicle. "Arda" in Tolkien means the world. The word is expansive, symmetrical, capable of throaty emotion or airy solemnity: it encompasses everything. It is marvellous that Nabokov came to the Russian name Ada, identically pronounced, to signify the same. He noted, in his own copy, that his book was "the pearl of American literature", and the judgement is unfair only in that it fails to acknowledge Ada's supremacy over all the triumphs in English since Paradise Lost. It treats a superimperial pair in a prose of fierce perfection and harlequin variety. It includes a world of fatidic interest equal to that of Pale Fire, though its breadth is far greater. Pattern and shade in the happy hum of dwindling memory is coupled with the verve of original bliss to produce a hugy and tessellated reading experience. One is left feeling yokellish awe at this most successful of books, though Nabokov's personality is too intrinsic to Ada for it to be his greatest. For that, one returns to Pale Fire, in which Nabokov somehow achieved John Shade.