homonym –noun
1. homophone (def. 1).
2. a word the same as another in sound and spelling but different in meaning, as chase “to pursue” and chase “to ornament metal.”
vain –adjective, -er, -est.
1. excessively proud of or concerned about one's own appearance, qualities, achievements, etc.; conceited: a vain dandy.
2. proceeding from or showing personal vanity: vain remarks.
3. ineffectual or unsuccessful; futile: a vain effort. 4. without real significance, value, or importance; baseless or worthless: vain pageantry; vain display.
5. Archaic. senseless or foolish.
—Idiom
6. in vain, a. without effect or avail; to no purpose: to apologize in vain. b. in an improper or irreverent manner: to take God's name in vain.