A word ladder is a type of word puzzle in which you transform one word into another by changing one letter at a time, with each intermediate step forming a valid word. The starting and ending words must be of the same length, and each step in the sequence must result in a real word found in the dictionary. For example, changing "COLD" to "WARM" could go through "CORD," "CARD," and "WARD," with each word differing by just one letter from the previous one.

This puzzle was invented by Lewis Carroll and is also known by other names such as Doublets, word-links, laddergrams, or word golf. The main rule is that only one letter can be changed at each step, and every word in the sequence must be valid.