| Definition -
A game in which a given word is changed into another given word by a series of single letter changes that must always form another word.
For example, if the starting word is pig and the ending word is sty, the following is a successful sequence (called a word ladder): pig, wig, wag, way, say, and sty.
Etymology - The name was coined by Lewis Carroll, who invented the game the 1870s. It ultimately derives from the French doublet, a diminutive of double.
Note: Carroll called the two starter words doublets.
Oxford English Dictionary - The word's first citation in this sense is from 1879:
"The word ‘head’ may be changed into ‘tail’ by interposing the words ‘heal, teal, tell, tall’. I call the two given words a ‘Doublet’. … The easiest ‘Doublets’ are those in which the consonants in one word answer to consonants in the other, and the vowels to vowels."
(Lewis Carroll in Vanity Fair, 29 March 1879, 185/2)