A daffynition is a pun format involving the reinterpretation of an existing word, on the basis that it sounds like another word (or group of words). They are similar to transpositional puns, but often much less complex and easier to create.
As a result of this, daffynitions are popular with children. Otherwise they are considered a less dignified form of the pun, if such a thing can exist.
Examples
See also
Wiktionary:Daffynition
Transpositional pun