Heterogamy has a number of biological definitions:
In reproductive biology, heterogamy is the alternation of differently organized generations, applied to the alternation between parthenogenetic and a sexual generation. This type of heterogamy occurs for example in some aphids.
In cell biology, heterogamy is a synonym of anisogamy, the condition of having differently sized male and female gametes produced by different sexes or mating types in a species.
In botany, a plant is heterogamous when it carries at least two different types of flowers in regards to their reproductive structures, for example male and female flowers or bisexual and female flowers. Stamens and carpels are not regularly present in each flower or floret.
In a social context, heterogamy refers to a marriage between two individuals that differ in a certain criterion. For example, ethnic heterogamy refers to marriages involving individuals of different ethnic groups, and age-heterogamy refers to marriages involving partners of significantly different ages.