serious

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Careful in thought, full of concern, or restrained and dignified in manner; somber or grave.
• adjective Requiring or carried out with careful thought or concern.
• adjective Intended for sophisticated people.
• adjective Deeply interested or involved.
• adjective Concerned with important rather than trivial matters.
• adjective Not joking or trifling.
• adjective Informal Of considerable size or scope; substantial.
• adjective Being of such import as to cause anxiety.
• adjective Not easily answered or solved.

from The Century Dictionary.

• Grave in feeling, manner, or disposition; solemn; earnest; not light, gay, or volatile; of things, springing from, expressing, or inducing gravity or earnestness of feeling.
• In earnest; not jesting or making pretense.
• Important; weighty; not trifling.
• Attended with danger; giving rise to apprehension: as, a serious illness.
• Deeply impressed with the importance of religion; making profession of or pretension to religion.
• Synonyms Solemn, etc. See grave.
• 1 and Sedate, staid, sober, earnest.
• Great, momentous.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Grave in manner or disposition; earnest; thoughtful; solemn; not light, gay, or volatile.
• adjective Really intending what is said; being in earnest; not jesting or deceiving.
• adjective Important; weighty; not trifling; grave.
• adjective Hence, giving rise to apprehension; attended with danger.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Without humor or expression of happiness; grave in manner or disposition; earnest; thoughtful; solemn.
• adjective Important; weighty; not trifling; leaving no room for play; needing great attention; critical.
• adjective Really intending what is said; being in earnest; not jesting or deceiving; meaningful.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective completely lacking in playfulness
• adjective concerned with work or important matters rather than play or trivialities
• adjective requiring effort or concentration; complex and not easy to answer or solve
• adjective of great consequence
• adjective appealing to the mind
• adjective causing fear or anxiety by threatening great harm