set phrase

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

• noun grammar A common expression whose wording is not subject to variation.
• noun grammar A common expression whose words cannot be replaced by synonymous words without compromising the meaning.

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

• noun an expression whose meanings cannot be inferred from the meanings of the words that make it up