rhyming slang

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Slang in which a word is replaced by a word or phrase that rhymes with it, as mate by China plate, often with the rhyming word being dropped from the phrase.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any system of slang in which a word is replaced with a phrase that rhymes with it; the rhyming word often being dropped.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun slang that replaces words with rhyming words or expressions and then typically omits the rhyming component