Semelparity and iteroparity are two reproductive strategies available to living organisms. A species is semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles. Iteroparity can be further divided into continuous iteroparity (e.g., primates, including humans and chimpanzees) and seasonal iteroparity (e.g. birds, dogs, etc.). Some botanists use the parallel terms monocarpy and polycarpy.

In truly semelparous species, death after reproduction is part of an overall strategy that includes putting all available resources into maximizing reproduction, at the expense of future life (see § Trade-offs). In any iteroparous population there will be some individuals who happen to die after their first and before any second reproductive episode, but unless this is part of a syndrome of programmed death after reproduction, this would not be called "semelparity".

This distinction is also related to the difference between annual and perennial plants: An annual is a plant that completes its life cycle in a single season, and is usually semelparous. Perennials live for more than one season and are usually (but not always) iteroparous.

Semelparity and iteroparity are not, strictly speaking, alternative strategies, but end points along a continuum of possible modes of reproduction, although such intermediates are rare. Many organisms considered to be semelparous can, under certain conditions, survive their first bout of reproduction to reproduce two or more times.

The word "semelparity" was coined by evolutionary biologist Lamont Cole, and comes from the Latin semel ('once, a single time') and pario ('to beget'). This differs from iteroparity in that iteroparous species are able to have multiple reproductive cycles and therefore can mate more than once in their lifetime. Semelparity is also known as "big bang" reproduction, since the single reproductive event of semelparous organisms is usually large as well as fatal. A classic example of a semelparous organism is (most) Pacific salmon (Oncorhynchus spp.), which live for many years in the ocean before swimming to the freshwater stream of its birth, spawning, and dying. Other semelparous animals include many insects, including some species of butterflies, cicadas, and mayflies, many arachnids, and some molluscs such as some species of squid and octopus.

Semelparity also occurs in smelt and capelin, but other than bony fish it is a very rare strategy in vertebrates. In amphibians, it is known only among some Hyla frogs including the gladiator frog;^{[full citation needed]} in reptiles only a few lizards such as Labord's chameleon of southwestern Madagascar, Sceloporus bicanthalis of the high mountains of Mexico, and some species of Ichnotropis from dry savanna areas of Africa. Among mammals, it exists only in a few didelphid and dasyurid marsupials. Annual plants, including all grain crops and most domestic vegetables, are semelparous. Long-lived semelparous plants include century plant (agave), Lobelia telekii, and some species of bamboo.

This form of lifestyle is consistent with r-selected strategies as many offspring are produced and there is low parental input, as one or both parents die after mating. In semelaparous mammals, all of the male's energy is diverting into mating and the immune system is repressed. High levels of corticosteroids are sustained over long periods of time. This triggers immune and inflammatory system failure and gastrointestinal hemorrhage, which eventually leads to death.

The term iteroparity comes from the Latin itero, to repeat, and pario, to beget. An example of an iteroparous organism is a human—humans are biologically capable of having offspring many times over the course of their lives.