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Anisogamy is sexual reproduction involving two classes of gametes that differ in size. The larger gamete defines the female reproductive role, and the smaller gamete the male role. In humans, an ovum and a sperm illustrate the distinction. Mobility is a separate character: the definition is not simply that one gamete moves and the other does not.

A resource-budget model makes the evolutionary tension visible. With total investment $B$ and mass $m$ per gamete, the number produced is $B/m$. Smaller gametes can be more numerous, whereas a larger combined contribution can improve survival of the resulting zygote. Models ask when these competing effects favor distinct size classes rather than a single intermediate size; the answer depends on their assumptions about encounters and biological fitness.^[ref]

G. A. Parker coauthored a 1972 evolutionary model for the origin of anisogamy, the difference between small and large gametes. The model connected the number of gametes a producer can make with the resources available to a resulting zygote. Producing many small gametes can improve opportunities for fusion, whereas larger gametes can improve provisioning; these pressures need not favor one intermediate size.^[ref]

The contribution made the origin of two gamete-size strategies a problem in natural selection. It explained a possible mechanism under explicit reproductive assumptions, rather than making every observed sex difference a necessary consequence of anisogamy.

An ovum is the female gamete. In an anisogamy system it is the larger gamete type, generally contributing substantial cytoplasm as well as a genetic contribution to the zygote.

An oocyte is a developing female germ biological cell; its stage of meiosis matters when describing the gamete precisely. Oogenesis often partitions most of the cytoplasm into one large product while small polar bodies receive the other chromosome complements.

A sperm is a male gamete, generally the smaller gamete type in an anisogamy system. During fertilization, its genetic contribution combines with that of an ovum to form a zygote.

A sperm is a biological cell, whereas semen is a fluid mixture that can contain many sperm. Movement and delivery differ among lineages: water-dependent swimming occurs in many non-seed plants, while a pollen tube supplies a different delivery route in angiosperms.