A word chain, also known as a word game or word ladder, is a game where players create a sequence of words. The sequence must follow specific rules, with the most common rule being that each word differs from the previous one by only one letter.

Here's a basic example:

1. CAT
2. COT
3. COW

In this example, each word in the chain is created by changing one letter from the previous word.

Other common variations exist where words must start with the ending letter of the word before it.

For example:

1. Dog
2. Golf
3. Frog

Word chains can also be created based on rhymes, or other more complicated rules, but the most basic concept is to link words together in a chain based on defined parameters.