Games : What is a turn?

ToME 4 uses both turns and action speed in its terminology, and it's confusing me.

Is one turn a set amount of time (e.g. 2.6 seconds), an abstract "as long as it takes one to act" kind of deal, or something else entirely? This is extremely important, because it heavily affects how I should interact with timed effects.

For example, if I get a buff that lasts 10 turns, will increasing my action speed burn through the effect faster (because I take my 10 actions sooner), or get me more its use (because I can take more actions over the effect's fixed duration)?