The word "constellation" comes from the Old French "constellacion," which comes from the Late Latin "constellationem."
Here's a breakdown of the word and its Latin roots:
con- Meaning "with" or "together." This describes the aspect of constellations as being gatherings or patterns of stars.
stella- Stella or stellae is the word for stars, but a long time ago the same word was used to described planets or other celestial bodies. This word pre-dates technology which allows us to look into the sky and distinguish between stars and planets.
-tion- This is a suffix for words of Latin origin which describes an action or state of being.
So, altogether, constellation roughly means "stars which are clustered together."