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') WordNet 3.0 & Open English WordNet

A dead metaphor is a figure of speech which has lost the original imagery of its meaning by extensive, repetitive, and popular usage, or because it refers to an obsolete technology or forgotten custom. Wikipedia

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. Wiktionary