What term describes a close relationship between organisms of different species, benefiting at least one organism?

The term that describes a close relationship between organisms of different species, benefiting at least one organism, is symbiosis. Symbiosis encompasses a broad range of interactions between species, which can include mutually beneficial relationships, as well as ones that may benefit only one species at the expense of the other or benefit one without affecting the other.

Within the context of symbiosis, there are specific types that fall under this umbrella, such as parasitism, where one organism benefits at the expense of another, and mutualism, where both organisms derive benefit. Commensalism is another type of symbiotic relationship where one organism benefits while the other is neither helped nor harmed. Symbiosis as a term captures all these types of interactions under its definition, making it the most inclusive and appropriate choice for describing a close relationship that benefits at least one organism.