Is the word on a preposition?

Answer 1

Yes, on can be a preposition.

A quick test for a preposition (which the word onpasses):

Can you form a phrase with a preposition and noun, along the lines of under the table or in the end?

You can do this with on:

on the chair, on the day they met.

Note that the word "on" can also act as an adjective or adverb:

the television is on

he turned the light on

Answer 2

No, "on" is a preposition.