A close and often long-term interaction between two different biological species.

Here's why:

* Close and Long-Term: Symbiosis implies a strong and sustained relationship between the two species, not just a fleeting encounter.

* Different Species: It specifically refers to interactions between organisms belonging to different species.

* Interaction: Symbiosis focuses on the relationship itself, the way the two species affect each other. This can be beneficial, harmful, or neutral for one or both species.

It's important to note that symbiosis is a broad term, and there are different types of symbiotic relationships:

* Mutualism: Both species benefit from the interaction.

* Commensalism: One species benefits, while the other is neither harmed nor helped.

* Parasitism: One species benefits at the expense of the other.

So, while symbiosis is a broad term, it always refers to the close and long-term interaction between two different species, regardless of the specific nature of their relationship.