Severe
Part of speech: adjective
Definition: Very bad or intense. Strict or harsh.
Example sentence: Adversity has the same effect on a man that severe training has on the pugilist: it reduces him to his fighting weight.
Serious
Part of speech: adjective
Definition: Without humor or expression of happiness; grave in manner or disposition; earnest; thoughtful; solemn.Important; weighty; not trifling; leaving no room for play; needing great attention; critical.Really intending what is said; being in earnest; not jesting or deceiving; meaningful.
Example sentence: My work is a game, a very serious game.