dead metaphor

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

• noun A figure of speech whose metaphorical meaning has become so familiar over time that its literal meaning is forgotten or goes unnoticed, as in arrive at a conclusion or cast a glance.

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

• noun linguistics A former metaphor which has in effect lost its metaphorical status and become literal, e.g. "electric current" (electricity was at first thought to be analogous to water). Not to be confused with stale metaphor (a type of cliché), although it often is.

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

• noun a metaphor that has occurred so often that it has become a new meaning of the expression (e.g., `he is a snake' may once have been a metaphor but after years of use it has died and become a new sense of the word `snake')