From St.Thomas Aquinas, Summa Theologica

Accordingly, the first change wrought in the
appetite by the appetible object is called "love," and is nothing else than
complacency in that object; and from this complacency results a movement
towards that same object, and this movement is "desire"; and lastly, there is rest
which is "joy." Since, therefore, love consists in a change wrought in the appetite
by the appetible object, it is evident that love is a passion: properly so called,
according as it is in the concupiscible faculty; in a wider and extended sense,
according as it is in the will.