Twin vs Gemini

Twin and Gemini both are nouns.

Word	Noun	Adjective	Verb	Adverb
Twin	Yes	No	No	No
Gemini	Yes	No	No	No

As nouns, Twin and Gemini are synonyms defined as:

Twin and Gemini: (astrology) a person who is born while the sun is in Gemini

Twin (noun)	Gemini (noun)
a waterfall in the Snake River in southern Idaho	the third sign of the zodiac; the sun is in this sign from about May 21 to June 20
(astrology) a person who is born while the sun is in Gemini	a zodiacal constellation in the Northern Hemisphere between Taurus and Cancer on the ecliptic
	(astrology) a person who is born while the sun is in Gemini