of, showing, or characterized by deep thought.

of grave or somber disposition, character, or manner

a serious occasion; a serious man.

being in earnest; sincere; not trifling

His interest was serious.

requiring thought, concentration, or application

serious reading; a serious task.

weighty or important

a serious book; Marriage is a serious matter.

giving cause for apprehension; critical

The plan has one serious flaw.

Medicine/Medical . (of a patient's condition) having unstable or otherwise abnormal vital signs and other unfavorable indicators, as loss of appetite and poor mobility

patient is acutely ill.

that which is of importance, grave, critical, or somber

You have to learn to separate the serious from the frivolous.

adjective

flippant , funny , happy , light , minor , trivial , unimportant , unserious

adjective

austere , bound , bound and determined , businesslike , cold sober , contemplative , deadpan * , deliberate , determined , downbeat * , earnest , funereal , genuine , go for broke , grave , grim , honest , intent , long-faced , meditative , no-nonsense * , pensive , pokerfaced , reflective , resolute , resolved , sedate , set , severe , sincere , sober , solemn , staid , steady , stern , thoughtful , unhumorous , unsmiling , weighty , arduous , dangerous , deep , difficult , far-reaching , fateful , fell , formidable , grievous , hard , heavy , important , laborious , major , meaning business , meaningful , menacing , momentous , no joke , no laughing matter , of consequence , operose , out for blood , playing hard ball , pressing , significant , smoking , sobering , strenuous , strictly business , threatening , tough , ugly , unamusing , urgent , worrying , somber , no-nonsense , sobersided , tall , uphill , alarming , critical , crucial , demure , dire , ernest , humorless , intense , life and death , saturnine