Donald Ian RankinDonald Ian Rankin's Journal
1) The wrong definition
Moderate: One whose views the I, the speaker, consider to be reasonable and sensible.
This definition is obvious absurd, but it's still used fairly widely. In particular, it's beloved by those who don't fit the other two definitions, but still want to lay claim to a politically-valuable term and brand their opponents with the equally loaded "extreme". When I see someone using the word in this way, I tend to take it as a sign that they're not worth taking terribly seriously.
2) The objective but context-dependent definition:
Moderate: One whose views are opposed to a comparable (not necessarily near-equal, but not orders of magnitude apart) extent from either direction.
In most cases, this is much the most sensible definition. It captures most of what one wants to say about moderation and extremism, and it's fairly objective. The big drawback is that you need to define what group of people you're talking about moderation with respect to - a position that is moderate in the politics of one country may be extreme in the politics of another.
3) The hand-wavy but context-independent psychological definition:
Moderate: the psychological opposite of a crusader.
In some ways, this is nearly as spurious as the first definition - in particular, it's far harder to measure than the second - but I think it does have some validity. There does appear to be a meaningful psychological split between those who view politics as a righteous crusade and those who view it as an exercise in problem-solving. Notably, the two sorts tend to heavily distrust one another even when their actual views are not that dissimilar (I think this is a large part of the reason why the Sanders/Clinton wars in GD-P are so bitter). And this psychological divide appears to correlate non-trivially (although far from perfectly) with the second possible definition, and has the advantage of transferring better between countries.