While on metaphor watch, certain phrases and ideas will recur. Metaphor is one of the broadest figures of speech. I use the Concise Oxford Dictionary (COD), which defines it as a word or phrase applied to something to which it is not literally applicable. That is a wide definition, and other authorities give tighter ones: for example, Merriam-Webster defines it as one kind of object or idea used in place of another, to suggest a similarity. Metaphor is idiomatic use of a language. The COD has two definitions for idiom: a group of words whose meaning is not deducible from the words’ individual meanings, and a form of expression natural to a language or group. Two things follow, one directly and one indirectly: metaphor is necessarily idiomatic (although not all idiom is metaphor); and metaphor, in common with all idiom, can be difficult for readers who do not have English as their first language (commonly referred to as EAL: English as an additional language), or who come from a different culture.

Thus game, set and match is an idiomatic expression, and is a metaphor from tennis, meaning that something has been definitely decided: this is the position; that is what happens. Set about (if not literal: they were set about 1 cm apart) is idiom, and could mean to start (they set about solving the problem) or assault (they set about him with a hammer). Game, set and match will translate into any language whose culture includes tennis; but set about is unlikely to translate directly in any language.

Idiom just is: it is language established by usage. Metaphors have a purpose: they explain and enliven. Metaphor watch does not condemn metaphors, but asks they be used with care and consideration. Explanation of a difficult idea by the metaphorical application of a simpler one is useful, as long as it remains realised that it is only a starting point. Describing a gene as a blueprint is fine, as far as it goes; but a gene is not really anything like a blueprint. Enlivening text with metaphor is also useful, if it keeps the reader reading, but the dangers are exaggeration or ineptness (by the writer) and boredom or derision (from the reader).

Metaphors have a life: they start as fresh expressions in the writer’s mind, but they get tired, and eventually become clichés: phrases that are overused and show a lack of original thought. There is no firm boundary between metaphor and cliché, partly it is in the mind of the reader. The advice ‘to avoid clichés like the plague’ is itself a cliché, which has become a meta-cliché, because the statement The advice ‘to avoid clichés like the plague’ is a cliché is itself a cliché.

Competing interests: I have read and understood BMJ policy on declaration of interests and declare that my only competing interest is my co-authorship of a book about medical English.

Neville Goodman is a retired consultant anaesthetist and a writer, and co-author of a book on medical English.