Part of Speech: Noun

Marriage: the state of being united as spouses in a consensual and contractual relationship recognized by law

Source: Webster`s Dictionary

Found in, “Sonnet 116”  (line 1), “Let me not to the marriage of true minds”

The word marriage means the relation between two married people, the poet uses the word “minds” in the same line instead of anything more base, like “hearts” which might be to let us know that this perfect love is the partnership of two individuals of great thinking,  who aren’t simply driven by emotions or hormones.