Definition of on:

part of speech: pronoun

One; they.

part of speech: adverb

Forward in progression; in continuance; without ceasing; opposed to off; on a sudden, suddenly.

part of speech: preposition

Above, and in contact with the upper surface of a thing; not off; upon; at or near; denoting dependence or reliance; denoting nearness or closeness; denoting addition or accumulation.

part of speech: interjection

A word of excitement or encouragement to attack or advance; a contraction for go on.