1. An insane or excessive jealousy.
2. An excessive zeal or a passionate pursuit of a cause, to the point of causing illness.
3. Excessive zeal, carried to the point of morbidity, in the advocacy of any cause.
4. A morbid perseverance and energy in working on a project; especially, a political or religious activity.

A form of monomania sometimes manifesting itself in over zeal in attempts to gain supporters to some public cause.

5. Etymology: from Greek zelotypia, "rivalry, envy"; from zelos, "zeal", + typto, "to strike". If you put the two words together, you get "strike (with) zeal".