By itself, metaphysical means dealing with the relationship between spirit to matter or the ultimate nature of reality. The Metaphysical poets are obviously not the only poets to deal with this subject matter, so there are a number of other qualities involved as well:
This argument is answered in at least two ways:
During the Renaissance, Plato got mingled with Christian and Eastern thought. Through this mingling we get
Platonic love (which is a lot more than you probably think it means). For Plato, beauty proceeds in a series of steps from the love of one beautiful body to that of two, to the love of physical beauty in general, and ultimately to the love of that beauty "not in the likeness of a face or hands or in the forms of speech or knowledge or animal or particular thing in time or place, but beauty absolute, separate, simple, everlasting--the source and cause of all that perishing beauty of all other things."
When this scheme is Christianized by equating this ultimate beauty with the Divine Beauty of God, the Renaissance Platonic lover can move in stages through the desire for his mistress, whose beauty he recognizes as an emanation of God's, to the worship of the Divine itself.
This complex doctrine of love which embraces sexuality (the mystical union of souls, cf. Donne's "The Canonization") but which is directed to an ideal end (discussed in Plato's Symposium) is particularly evident in Donne. (But we see it in poets from Sidney to Lawrence).
Platonic love has also come to mean a love between
individuals which transcends sexual desire and attains spiritual heights
(for examples, see some of the courtly romances like Tennyson's Idylls
of the King), as well as homosexual love (see Forster's Maurice),
derived from the praise of homosexual love in The Symposium.
The term "Metaphysical Poet" was first coined by the critic Samuel Johnson (1709-1784) and he used it as a disparaging term. Earlier, John Dryden had also been critical of the group of poets he grouped together as too proud of their wit. Johnson and Dryden valued the clarity, restraint and shapeliness of the poets of Augustan Rome (which is why some 18th century poets are called "Augustan," and therefore were antagonistic towards poets of the mid-17th century.
The Metaphysicals were out of critical favor for the 18th and 19th centuries (obviously, the Romantic poets found little in this heavily intellectualized poetry). At the end of the 19th century and in the beginning of the 20th century, interest in this group picked up, and especially important was T.S. Eliot's famous essay "The Metaphysical Poets" (1921). Interest peaked this century with the New Critics school around mid-century, and now is tempering off a bit, though Donne, the original "Big Name" is being superceded now by interest in George Herbert, who's religious seeking and questioning seems to be hitting a critical nerve.