serious

degrees more serious - most serious

adjective: thoughtful, grave, or solemn in manner or character; not lighthearted or frivolous
- She had a serious expression as she delivered the important news.
adjective: demanding or characterized by careful consideration or application; significant or important
- He is a serious candidate for the promotion.
adjective: sincere and meaning what one says; not joking or deceiving
- Are you serious about moving to another country?
adjective: extensive or worrying in degree, extent, or importance; severe
- The patient has a serious illness.