Definition:

A familiar comparison that does not call attention to itself as a figure of speech. Contrast with creative metaphor.

George Lakoff points out that many of "the metaphorical expressions discussed in the literature on conventional metaphor are idioms" ("The Contemporary Theory of Metaphor," 1993).

See Examples and Observations, below. Also see:

• Metaphor
• Dead Metaphor
• Primary Metaphor
• What Is a Metaphor?

Examples and Observations:

• "Creative metaphors contrast with conventional metaphors. These are metaphorical usages which are found again and again to refer to a particular thing. Cases in point are the metaphors of cells fighting off infection and of micro-organisms invading; and the metaphorical meaning of divorced to mean 'completely separated' and field to refer to a specialized subject or activity. These kinds of metaphors are institutionalized as part of the language. Most of the time we hardly notice them at all, and do not think of them as metaphorical when we use or encounter them."
  (M. Knowles and R. Moon, Introducing Metaphor. Routledge, 2006)
• "Conventional metaphors are embedded in our culture to the point that we literally interpret their meaning. The example, "time is money," is a conventional metaphor that has become embedded in American culture. We understand 'time' in terms of money and conceptualize 'time' as being 'spent,' 'saved,' or 'wasted.' Such basic conventional metaphors help structure our everyday thinking. We interpret these metaphors literally as a conventional part of speech, and this common language further influences how we conceptualize and behave. For example, 'argument is war' formulates how we think about arguing. We 'defend,' 'strategize,' 'attack,' and 'defeat' arguments."
  (Donna Cox, "Metaphoric Mappings: The Art of Visualization." Aesthetic Computing, ed. by Paul A. Fishwick. The MIT Press, 2006)

• "Equating cars with humans or animals would be considered a conventional metaphor as can be seen by the abundance of similar metaphors and idioms. For example, if we are exhausted we might say 'I just ran out of gas' and cars that have low gas mileage are called gas guzzlers."
  (Eileen Cornell Way, Knowledge Representation and Metaphor. Kluwer Academic Publishers, 1991)
• MORE IS UP
  "Perhaps the deepest question that any theory of metaphor must answer is this: why do we have the conventional metaphors that we have? . . .
  
  "Take a simple case: the MORE IS UP metaphor, as seen in expressions like prices rose; his income went down; unemployment is up; exports are down; the number of homeless people is very high.
  
  "There are other languages in which MORE IS UP and LESS IS DOWN, but none in which the reverse is true, where MORE IS DOWN and LESS IS UP. Why not? Contemporary theory postulates that the MORE IS UP metaphor is grounded in experience--in the common experiences of pouring more fluid into a container and seeing the level go up, or adding more things to a pile and seeing the pile get higher. These are thoroughly pervasive experiences; we encounter them every day in our lives."
  (George Lakoff, "The Contemporary Theory of Metaphor." Metaphor and Thought, ed. by A. Ortony. Cambridge University Press, 1993)