Conceit is a figure of speech where two immensely different things are joined together by similes or metaphors. Conceit creates unlikely comparisons that enable readers to see things in a new and surprising way. The literary device often uses an extended metaphor to create an elaborate comparison in a large section of a poem or the entire poem.
example: "Sonnet 97" by William Shakespeare
"How like a winter hath my absence been
From thee, the pleasure of the fleeting year!
What freezings have I felt, what dark days seen!
What old December's bareness everywhere!
And yet this time remov'd was summer's time,
The teeming autumn, big with rich increase,
Bearing the wanton burthen of the prime,
Like widow'd wombs after their lords' decease:
Yet this abundant issue seem'd to me
But hope of orphans and unfather'd fruit;
For summer and his pleasures wait on thee,
And thou away, the very birds are mute;
Or if they sing, 'tis with so dull a cheer
That leaves look pale, dreading the winter's near."
(Winter is compared to his separation from his lover)