Dictionary

Without humor or expression of happiness; grave in manner or disposition

Example: It was a surprise to see the captain, who had always seemed so serious, laugh so heartily.

Important; weighty; not insignificant

Example: This is a serious problem. We'll need our best experts.

Really intending what is said (or planned, etc); in earnest; not jocular or deceiving

Example: After all these years, we're finally getting serious attention.

(of a relationship) Committed.

In a serious manner; seriously.