A conceit, which can be either metaphor or simile, is a comparison of two things radically and startlingly unlike—in Samuel Johnson’s words, “yoked by violence together.” A conceit is as far removed as possible from the purely sensuous comparison of “the eyes of the potato.” It compares two things that have very little or no immediately apprehensible similarity, and so it is the nature of the conceit to be long. The author must explain to us, sometimes at great length, why these things can be said to be alike. When John Donne compares a flea to the Holy Trinity, the two images have no areas of reference in common, and we don’t understand. He must explain to us that the flea, having bitten both the poet and his lover, now has the blood of three souls in its body.

Janet Burroway, Writing Fiction, Tenth Edit…, loc. 597