Synonyms for Serious:

adj.

adult (adjective)
full-grown, of age, virile, adult, reasonable, nubile, mature, developed.
critical (adjective)
pivotal.
crucial, weighty (adjective)
difficult, dangerous, meaningful, arduous, grievous, deep, pressing, tough, laborious, momentous, major, hard, sobering, significant, important.
funereal (adjective)
funereal.
important (adjective)
important, momentous, salient, heavyweight, influential, notable, central, big-name, high-powered, fundamental, big, critical, substantial, earthshaking, distinguished, high, major, considerable, big-wig, crucial, grave, vital, eminent, pivotal, prominent, significant, meaningful, consequential, key, noteworthy, world-shaking.
life-and-death (adjective)
earthshaking.
resolute (adjective)
devoted, deliberate, tenacious, spunky, tireless, persistent, staunch, zealous, reliable, courageous, dauntless, bold, resolved, persevering, valiant, insistent, unwavering, strong-willed, committed, dedicated, plucky, thorough, relentless, unchangeable, unyielding, resolute, bullheaded, constant, intent, indomitable, iron-willed, strong-minded, ruthless, diligent, unshakeable, steadfast, loyal, true, determined, steady, inexorable, indefatigable, steely, unbending, decisive, implacable, earnest, willing, faithful, uncompromising, stubborn, unflinching, dogged, stern, headstrong, tough, decided, firm, set, hearty, fearless.
severe (adjective)
severe.
somber, humorless (adjective)
solemn, sedate, deliberate, staid, sincere, severe, grave, set, determined, thoughtful, intent, stern, sober, steady, resolved, funereal, earnest, resolute, no-nonsense, businesslike.

n.

large, enormous, pressing, widely, extensive, high, big, huge, great. actively, complicit, be harnessed to, staid, in the process of doing something, engaged, active, attitude, involved, submerged, concerned. ruinous, damaging, sobersided, work, disastrous, injurious, businesslike, no-nonsense, harmful. anxious, upset, worried, troubled, on edge, nervous, alarmed, frightened, tense, uneasy, help. cohabit, fool around, go together, carry on, court, cheat, date, go with, dally with, go out. humorlessly, officious, funereal, substantial, deep, stolid.

Other synonyms:

involved, stolid, complicit, actively, submerged, sobersided, officious, engaged. staid, tall, concerned, uphill, arduous, tense. anxious, laborious. active. deep. injurious
harmful.
matter of fact
no-nonsense.
purposeful
businesslike.

Usage examples for serious

  1. When, after some time, he returned, he was very serious – John Halifax, Gentleman by Dinah Maria Mulock Craik
  2. " I be for London town," he said to himself with a serious look. – A Boy's Ride by Gulielma Zollinger
  3. Well, said Denry, I hope it's not serious – The Card, A Story Of Adventure In The Five Towns by Arnold Bennett