Definitions of serious:

• adjective:   of great consequence

  Example: "Marriage is a serious matter"

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

  Example: "A serious student of history"

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

  Example: "Raised serious objections to the proposal"

• adjective:   appealing to the mind

  Example: "A serious book"

• adjective:   completely lacking in playfulness
• adjective:   causing fear or anxiety by threatening great harm

  Example: "A serious wound"