The term "metaphysical conceit" refers to an extended metaphor or comparison that is characteristic of the works of the seventeenth century Metaphysical Poets, a group including John Donne, Andrew Marvell, and George Herbert. The conceits, as the school of poetry with which they are associated, are defined by being highly wrought, combining the intellectual (often with a strong tincture of philosophy or natural science) with the emotional, and strikingly unusual, bringing together things one would ordinarily not compare, in immediate contrast to the more conventional metaphors used by the Petrarchan poets of the 16th century.
A typical example is Donne's use of the flea as a metaphor for sexual congress:
O 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.