conceit

In literature, an elaborate and, sometimes, far-fetched image, which extends a metaphor into as many layers of meaning as it will bear.

Conceits thrive on relating apparently impossible objects or emotions. Shakespeare's Richard II attempts to compare his prison cell with the world. John Donne compares an icy garden to his frozen feelings after a separation from his lover.

Conceit also refers to an artistic device which has become so widely used it is conventional; using blurred or out-of-focus filming techniques to denote a memory sequence is a conceit; referring to a pool of water as a mirror can be traced through literature back to Greek mythology, and is known as a literary conceit.