Extended metaphor
A metaphor developed across several sentences or paragraphs, each element extending the initial image.
An extended metaphor carries an initial metaphor forward, deploying its lexical field across several sentences, a stanza, sometimes an entire text. If life is compared to a sea crossing, the extended metaphor will chain storms, headings, shipwreck and harbor — each new element belonging both to the image and to the subject.
It structures the text as much as it adorns it: in fiction it can unify a whole scene under one tonality; in poetry it is often the poem's very framework. The risk is mechanism — when the reader feels the author unrolling the image by system, the effect wears out.
Example
Life as a voyage: "I weighed anchor, weathered storms, and I am still looking for a harbor."
In the workshop
Extend the image as long as it illuminates; cut as soon as it merely decorates. Three well-placed echoes beat a full page of forced coherence.