Definition of "Serious" :

adjective: causing fear or anxiety by threatening great harm

"A serious wound."

adjective: requiring effort or concentration; complex and not easy to answer or solve

"Raised serious objections to the proposal."

adjective: of great consequence

"Marriage is a serious matter."

adjective: appealing to the mind

"A serious book."

adjective: concerned with work or important matters rather than play or trivialities

"A serious student of history."

adjective: completely lacking in playfulness