1. The condition, rule, or custom of being married to only one person at a time (opposite of polygamy or bigamy); chiefly applied to the rule or custom (more explicitly called monogyny) by which a man can have only one wife, but also including monandry, the rule or custom by which a woman can have only one husband.
2. The practice or principle of marrying only once, or of not remarrying after the death of the first spouse:; the opposite of digamy.