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.