levity

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

• noun Humor, merriment, or a lack of seriousness, especially when inappropriate.
• noun Archaic Inconstancy; changeableness.
• noun Archaic The state or quality of being light; buoyancy.

from The Century Dictionary.

• noun Lightness of weight; relatively small specific gravity.
• noun A tendency to rise by a force contrary to gravity.
• noun Lightness of spirit or temper.
• noun Carelessness of temper or conduct; want of seriousness; disposition to trifle; inconstancy; volatility: as, the levity of youth.
• noun Synonyms . Levity, Volatility, Flightiness, Frivolity, Lightness. All these words are founded upon the idea of the lack of physical and, by figure, of mental and moral substance or weight, with a resulting ease in flying away from what is wise. The first three refer especially to outward conduct. Levity is a want of seriousness, temporary or habitual, a disposition to trifle with important interests. Volatility is that moral defect by which one cannot dwell long upon any one object of thought, or turns quickly from one source of pleasure to another: the word does not convey much opprobrium; in the young some degree of volatility is expected. Flightiness borders upon the loss of sanity in caprice or excitement of fancy; it is volatility in an extreme degree. Frivolity is a matter of nature, an inability to care about any but the most petty and trifling things. Lightness is not so strong as frivolity, but covers nearly the same ground; it emphasizes inconstancy.

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

• noun The quality of weighing less than something else of equal bulk; relative lightness, especially as shown by rising through, or floating upon, a contiguous substance; buoyancy; -- opposed to gravity.
• noun Lack of gravity and earnestness in deportment or character; trifling gayety; frivolity; sportiveness; vanity.
• noun Lack of steadiness or constancy; disposition to change; fickleness; volatility.

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

• noun Lightness of manner or speech, frivolity
• noun obsolete Lack of steadiness
• noun The state or quality of being light, buoyancy
• noun countable A lighthearted or frivolous act

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

• noun a manner lacking seriousness
• noun feeling an inappropriate lack of seriousness