"The Definition of Love" begins with a "rare" birth—that is, a birth that's both unusual and glorious. The speaker's love (meaning not an actual beloved, but an emotion) was born from the conjunction of two unlikely parents: its father is "Despair," and its mother is "Impossibility."
In other words, this love was doomed from the start. From the very moment of its birth to those strange personified parents, this was a love that could never be realized; it was the kind of impossible love that might drive one to despair.
But this impossibility, the speaker will go on to say, is exactly what makes this love so powerful. The certainty that the speaker can never be with a beloved proves that love's intensity—and sharpens it. Despair doesn't kill the speaker's love, but creates it.
The speaker will reveal the "Definition of Love" in more than one sense, then. The poem will define love here as one defines any word: by showing what it is and what it means. But it will also explore its definition in the sense of its limitation, the boundaries that define it. This paradoxical brain-twister of a poem will find love in the place where the infinite meets the impossible.
Despite its complex ideas, "The Definition of Love" uses a deceptively simple form. Its eight quatrains (or four-line stanzas) are rhymed ABAB and written in iambic tetrameter (lines of four iambs, metrical feet with a da-DUM rhythm, as in "My love | is of | a birth | as rare"). This shape, straightforward as a folk song's, will frame a lofty, cerebral vision of impossible love.