adjective

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

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

2. Important; weighty; not insignificant

   “This is a serious problem. We'll need our best experts.”

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

   “After all these years, we're finally getting serious attention.”

Synonyms: earnest, solemn

adverb

1. In a serious manner; seriously.