Demanding careful consideration or application; important and requiring attention. Tax evasion is a serious offence under the law.
Solemn or thoughtful in character or manner. She gave him a serious look, indicating that she was not joking.
Significant or worrying because of possible danger or risk. The doctor said it was a serious condition that needed immediate attention.
Not joking or intended to be funny. He was serious when he said he would quit his job if things didn't improve.
Considerable in amount, degree, or intensity; substantial. They invested a serious amount of money in the project.
Showing sincere and earnest intention, commitment, or effort. She's a serious student who spends hours in the library every day.