A conceit is basically a simile, or a comparison between two dissimilar things. In literature, a conceit is an extended metaphor with a complex logic that governs a poetic passage or entire poem. By juxtaposing, usurping and manipulating images and ideas in surprising ways, a conceit invites the reader into a more sophisticated understanding of an object of comparison. Dr. Johnson pointed out that in metaphysical poetry, 'the most heterogeneous ideas are yoked by violence together'.
Via Sandra V. Barbosa