1. Grave in quality or manner.
2. Carried out in earnest: "He was engaged in serious drinking".
3. Deeply interested or involved: "She was a serious card player."
4. Designed for and addressing grave and earnest tastes; such as, serious art; serious music.
5. Not trifling or jesting: "We're serious. We expect you to complete the assignment on time."
6. Of considerable size or scope; substantial: "The city had a cleanup that cost serious money."
7. Of such character or quality as to appeal to the expert, the connoisseur, or the sophisticate: "Every serious kitchen needs at least one freezer."
8. Concerned with important rather than trivial matters.
9. Being of such importance so as to cause anxiety: "She had serious injuries."
10. Too complex to be easily answered or solved: "The governor raised some serious objections to the proposal."
11. In medicine, (of a patient's condition) having unstable or otherwise abnormal vital signs and other unfavorable indicators, as loss of appetite and poor mobility.