Symbiosis

I'm pretty sure that in 'O' Level Biology, I learnt that a symbiotic relationship is one where both species benefit. It appears that either my memory has let me down, or I learnt wrong, or the definition has changed.

Symbiosis is now defined (by North Carolina State University, for example, and also by our old friend Wikipedia) as "any interaction where two species live together in close physical association". If one species benefits and the other is harmed, the relationship is parasitic; if one species benefits, and the other doesn't benefit but is not harmed either, it's commensal; if both species benefit, it's mutualistic.

Parasitism I remember; but I don't remember being taught about commensalism or mutualism.

There is a fourth possibility as to why what I think I learnt in 'O' Level Biology appears to be wrong. It's that I learnt the British version, and the versions given by NCSU and Wikipedia are American.