WordNet sense 1 (apathy demonstrated by an absence of emotional reactions):

impassiveness, impassivity, phlegm, stolidity, emotionlessness, unemotionality

WordNet sense 2 (the trait of remaining calm and seeming not to care; a casual lack of concern):

nonchalance, unconcern

WordNet sense 3 (the trait of lacking enthusiasm for or interest in things generally):

apathy, numbness, spiritlessness