serious

Appealing to the mind.

“a serious book”

Completely lacking in playfulness.

Causing fear or anxiety by threatening great harm.

“a serious wound”

“a serious turn of events”

Concerned with work or important matters rather than play or trivialities.

“a serious student of history”

“a serious attempt to learn to ski”

Of great consequence.

“marriage is a serious matter”

Requiring effort or concentration; complex and not easy to answer or solve.

“raised serious objections to the proposal”

“the plan has a serious flaw”