(n.) a non-reciprocal metaphor
When a writer describes something in terms of something else, that’s a metaphor. But when that metaphor only works one way—in other words, the thing being described in terms of something else can’t be turned around to describe it in return—then that is an anacolutha.
Anacolutha, or anacoloutha, is a specific form of metaphor often known more simply as a ‘non-reciprocal metaphor’. So x might be used as a clever way of describing y, but y can’t feasibly or easily be used to describe x in return. The example we put out on Twitter to illustrate this fairly confusing word suits this template nicely: in Shakespeare’s Romeo and Juliet, Romeo famously describes his lips as being like “two blushing pilgrims”, flushed with excitement at the prospect of kissing their most admired treasure. Those two excited pilgrims, however, couldn’t easily be compared to lips in return, so this metaphor really only works one way.
A metaphor that does work reciprocally, meanwhile, is an acoloutha. So you might describe a safe or a locked chest as being like Fort Knox—and you could just as feasibly describe Fort Knox as being like a safe or a locked treasure chest, so the metaphor works both ways.
Confusingly, there is another entirely different trope in rhetoric known as an anacoluthon, or anacoluthia. It refers to the breaking or absence of the normal grammatical sequence of a sentence for rhetorical effect. It’s a common trope used in punchy newspaper headlines and news reports, that might read something like “England’s greatest playwright, Shakespeare—but is he the best in the world?”
Like the lion’s share of rhetorical terms, both the words anacolutha and anacoluthon are of Greek origin, and (for understandable reasons) both essentially mean ‘inconsistent’, ‘not following’, or ‘lacking sequence’. At their root is keleuthos, an old Greek word for a route or pathway, in the sense of the path through a standard grammatical sentence or linking one metaphor to another being broken.