Movement of from the cytoplasm, nucleus, or virion of one to that of another, where stable incorporation of that genetic material in the recipient can be viewed as a successful endpoint.
In other words, DNA, or less often RNA, has to move from being a portion of the hereditary material associated with one individual to being a portion of the hereditary material associated with a second individual. The resulting genome is a combination of the hereditary material sourced from two (or more) different parents.
This movement can be horizontal (between otherwise independent individuals) as well as vertical (from parent to offspring).
The following video considers sex with very much a though not only a human perspective:
At the point of successful incorporation of genetic material into the now hybrid sexual product, this new combination of genetic material is subject to natural selection. That is, in comparison with the parents, such hybrid progeny will either be more successful (higher fitness), less successful (lower fitness), or similarly successful.
It is one of the of sex, in other words, that unless an environment is changing, the fact that the parents were successful enough to reproduce – in combination with random change more often than not being detrimental rather than beneficial – suggests that the products of sex will more likely be less successful than their parents rather than more successful.
The word sex is also used synonymously with . See also sexual reproduction, asexual reproduction, meiosis, fertilization, bacterial sex, and .
Note that other processes of movement of genetic material from one individual to another exist, ones which are less strictly sexual since genetic material sourced from different parents does not (necessarily) end up into the same cell. These other processes include especially the movement of whole organisms such as symbionts between hosts, such as to form an organism's microbiome, though there too the movement can be distinguished into horizontal versus vertical. See also the concept of endosymbiosis.