A conceit is a type of extended metaphor used in literature, different from a regular metaphor in that the difference between the conceit and its intended meaning is extreme. The success of the conceit depends on the cleverness by which the comparison is made. Poets, John Donne and Emily Dickinson to name two, use them quite often with varying degrees of success. Samuel Johnson criticised them by saying that "the most heterogeneous ideas are yoked by violence together." Conceits may be brief or go on for pages.

One cannot just compare apples and oranges and have it be a conceit. Wit and cleverness are essential elements. To be successful, the reader must not be forced to expend too much intellectual effort to make the connection. Done well, they are useful for making complex or abstract ideas more intelligible. Enough talk. How about an example?

In the New Testament, in the Gospel of John, we find this verse:

I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world. (John 6, 51)
In this example, we see that bread is used as a conceit to show that Jesus provides a spiritual nourishment essential to the soul's eternal life, just as earthly bread is essential to the body's mortal life. It is obvious that bread and Jesus Christ are different things, but the comparison drives home the point that through him the spiritual life is sustained.

Gospel of John