Passion, fervor, ardor, enthusiasm, zeal denote intense, high-wrought emotion.
Passion implies an overwhelming or driving emotion; it may be either the most abstract or the most concrete of these terms. It may be used without implication of a specific emotion; thus, a poet without passion is a poet incapable of feeling or of displaying vehement, agitating, or soul-stirring emotion; to be in the grip of passion is to be swayed by violent emotion, but without a hint from the context the nature of the emotion remains unknown.
Fervor and ardor both imply the kindling of emotion to a high degree of heat, but fervor more often suggests a steady glow or burning and ardor a restless or leaping flame. Fervor is associated especially with matters (as emotions that express themselves in prayer, contemplation, or devotion) involving persistent warmth; ardor, with those (as emotions that express themselves in eager longings, or zealous efforts) that suggest the violence and sometimes the transitoriness or wavering of flames.
Enthusiasm often comes very close to ardor, but it may differ in its emphasis on the rational grounds for the emotion, such as thoroughgoing admiration for a person or thing or conviction of the worthiness of a cause or end.
Ardor may suggest aspiration without a clearly envisioned goal, but enthusiasm nearly always implies an objective, a cause, or an object of devotion; thus, a teacher may stimulate ardor in a pupil without necessarily directing the latter's emotion into a definite channel, but he stimulates enthusiasm only when he provides the pupil with something concrete to admire, to follow, or to fight for.
Zeal retains from earlier senses a suggestion of a goading or driving passion expressed as great ardor or enthusiasm for a cause or end and coupled with energetic and unflagging activity in the service of the cause or in the pursuit of the end.