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).
Daffynitions are popular with children, due to their ease of creation. Otherwise they are considered a less dignified form of the pun, if such a thing can exist.
Examples
Alarms: What an octopus is. (all arms) Dockyard: A physician's garden. (doc yard) Oboe: An English tramp. Pasteurise: Too far to see. Propaganda: A gentlemanly goose. (proper gander)see also Wiktionary:Daffynition