Defining "Emotion"

From Key Terms in Philosophy of Mind by Pete Mandik, to be published by Continuum in May 2010 (link to publisher's page).

emotion, an affective mental state, examples of which include states of fear, anger, disgust, and joy. Emotions are often regarded as obstacles to RATIONALITY, but they play central roles in quality of life, personal preferences and priorities, social affiliations, and morality. Emotions may be distinguished from other mental states such as judgment or BELIEF by the relative closeness of association between emotions and characteristic bodily reactions (e.g., increased heart rate, perspiration). Emotions may be differentiated from each other along numerous dimensions such as (1) the presence and type of intentional object (see INTENTIONALITY), (2) intensity, and (3) valence (positive versus negative). So, for an example concerning (1), joy and resentment
may be distinguished by what they are about or directed at, where resentment
is directed toward other people in a way that joy need not be. For an example concerning (2), irritation and rage may be distinguished by, among other things, their intensity. For an example concerning (3), joy and fear have opposite valences, with there being a relatively obvious sense in which one is more positive than the other.