adjective
1.
not light or frivolous (as in disposition, appearance, or manner)
he was disturbed by her stern, serious look
Synonyms: earnest, grave, no-nonsense, poker-faced, sedate, sober, sobersided, solemn, somber, staid, weighty
Related Word: businesslike, ||dern, determined, steady, steady-going; intent, serious-minded; contemplative, meditative, pensive, reflective, thoughtful; austere, severe, stern; humorless, unhumorous; grim
Idioms: serious as a judge
Contrasted words: flighty, ||flip, flippant, frivolous, volatile; casual, easy, relaxed
Antonyms: light, unserious
2.
expressing, involving, or characterized by seriousness or gravity (as of consequence)
a serious economic situation
Synonyms: grave, heavy, severe, weighty
Related Word: important, significant; sobering; unamusing, unfunny, unhumorous; grim
Idioms: no joke, no laughing matter
Contrasted words: inconsequential, insignificant, unimportant, unserious
Antonyms: trifling, trivial
3.
Synonyms: hard 6, arduous, difficult, formidable, heavy, laborious, operose, severe, strenuous, tough
4.
Synonyms: grave 3, dangerous, fell, grievous, major, ugly
Related Word: menacing, threatening