Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit from each other; amensalism, where one is harmed while the other is unaffected; and parasitism, where one is harmed and the other benefits.

The commensal (the species that benefits from the association) may obtain nutrients, shelter, support, or locomotion from the host species, which is substantially unaffected. The commensal relation is often between a larger host and a smaller commensal; the host organism is unmodified, whereas the commensal species may show great structural adaptation consistent with its habits, as in the remoras that ride attached to sharks and other fishes. Remoras feed on their hosts' fecal matter, while pilot fish feed on the leftovers of their hosts' meals. Numerous birds perch on bodies of large mammal herbivores or feed on the insects turned up by grazing mammals.

The word "commensalism" is derived from the word "commensal", meaning "eating at the same table" in human social interaction, which in turn comes through French from the Medieval Latin commensalis, meaning "sharing a table", from the prefix com-, meaning "together", and mensa, meaning "table" or "meal". Commensality, at the Universities of Oxford and Cambridge, refers to professors eating at the same table as students (as they live in the same "college").^{[citation needed]}

The Belgian zoologist and paleontologist Pierre-Joseph van Beneden introduced the term "commensalism" in 1876.

The commensal pathway was traversed by animals that fed on refuse around human habitats or by animals that preyed on other animals drawn to human camps. Those animals established a commensal relationship with humans in which the animals benefited but the humans received little benefit or harm. Those animals that were most capable of taking advantage of the resources associated with human camps would have been the 'tamer' individuals: less aggressive, with shorter fight-or-flight distances. Later, these animals developed closer social or economic bonds with humans and led to a domestic relationship.

The leap from a synanthropic population to a domestic one could only have taken place after the animals had progressed from anthropophily to habituation to commensalism and partnership, at which point the establishment of a reciprocal relationship between animal and human would have laid the foundation for domestication, including captivity and then human-controlled breeding. From this perspective, animal domestication is a coevolutionary process in which a population responds to selective pressure while adapting to a novel niche that includes another species with evolving behaviors.

The dog was the first domesticated animal and was domesticated and widely established across Eurasia before the end of the Pleistocene, well before the cultivation of crops or the domestication of other animals. The dog is often hypothesised to be a classic example of a domestic animal that likely traveled a commensal pathway into domestication. Archaeological evidence, such as the Bonn–Oberkassel dog dating to ~14,000 BP, supports the hypothesis that dog domestication preceded the emergence of agriculture and began close to the Last Glacial Maximum when hunter-gatherers preyed on megafauna.

The wolves more likely drawn to human camps were the less aggressive, subdominant pack members with lowered flight response, higher stress thresholds, and less wary around humans, and therefore better candidates for domestication. Proto-dogs might have taken advantage of carcasses left on site by early hunters, assisted in capturing prey, or provided defense from large competing predators at kills. However, the extent to which proto-domestic wolves could have become dependent on this way of life prior to domestication and without human provisioning is unclear and highly debated. In contrast, cats may have become fully dependent on a commensal lifestyle before being domesticated by preying on other commensal animals, such as rats and mice, without any human provisioning. The debate over the extent to which some wolves were commensal with humans before domestication stems from the debate over the level of human intentionality in domestication, which remains untested.