Emotion is a process, a particular kind of automatic appraisal influenced by our evolutionary and personal past, in which we sense that something is happening to our welfare, and a set of physiological changes and emotional behaviors begins to deal with the situation.
I like this because it pulls a few important elements of emotions that used to be disconnected in my mind into a crisp and coherent definition.
My own extrapolation: The "purpose" of an emotion then (if you can call it that) is to either (1) enact physiological changes to prepare us to deal with informational intrusions, or to (2) provide feedback that some event did or could make us better or worse off.
[This is not (of course) to say that emotions always serve their purpose well.]
