arena vs stadium

arena and stadium both are nouns.

Word	Noun	Adjective	Verb	Adverb
arena	Yes	No	No	No
stadium	Yes	No	No	No

As nouns, arena and stadium are synonyms defined as:

arena and stadium: a large structure for open-air sports or entertainments

Other synonyms of arena include bowl, sports stadium.

arena (noun)	stadium (noun)
a playing field where sports events take place	a large structure for open-air sports or entertainments
a large structure for open-air sports or entertainments
the central area of an ancient Roman amphitheater where contests and spectacles were held; especially an area that was strewn with sand
a particular environment or walk of life