via Wikipedia, Conceit:
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. Extended conceits in English are part of the poetic idiom of Mannerism, during the later sixteenth and early seventeenth century.
In English literature the term is generally associated with the 17th century metaphysical poets, an extension of contemporary usage. In the metaphysical conceit, metaphors have a much more purely conceptual, and thus tenuous, relationship between the things being compared. Helen Gardner observed that “a conceit is a comparison whose ingenuity is more striking than its justness” and that “a comparison becomes a conceit when we are made to concede likeness while being strongly conscious of unlikeness.” An example of the latter would be George Herbert’s “Praise,” in which the generosity of God is compared to a bottle which (“As we have boxes for the poor”) will take in an infinite amount of the speaker’s tears.
An often-cited example of the metaphysical conceit is the metaphor from John Donne’s “The Flea”, in which a flea that bites both the speaker and his lover becomes a conceit arguing that his lover has no reason to deny him sexually, although they are not married:
Oh stay! three lives in one flea spare
Where we almost, yea more than married are.
This flea is you and I, and this
Our marriage-bed and marriage-temple is.
When Sir Philip Sidney begins a sonnet with the conventional idiomatic expression “My true-love hath my heart and I have his”, but then takes the metaphor literally and teases out a number of literal possibilities and extravagantly playful conceptions in the exchange of hearts, the result is a fully formed conceit.