se·ri·ous
/ˈsirēəs/
adjective
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1.
(of a subject, state, or activity) demanding careful consideration or application.
"marriage is a serious matter"
Similar:
important
significant
consequential
of consequence
momentous
of moment
key
grave
weighty
far-reaching
major
urgent
pressing
crucial
critical
vital
life-and-death
high-priority
no joke
no laughing matter
careful
detailed
in-depth
deep
profound
meaningful
Opposite:
trivial
unimportant
superficial
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2.
acting or speaking sincerely and in earnest, rather than in a joking or halfhearted manner.
"actors who are serious about their work"