dense

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Having relatively high density.
2. adj. Crowded closely together; compact: a dense population.
3. adj. Hard to penetrate; thick: a dense jungle.
4. adj. Permitting little light to pass through, because of compactness of matter: dense glass; a dense fog.
5. adj. Opaque, with good contrast between light and dark areas. Used of a photographic negative.
6. adj. Difficult to understand because of complexity or obscurity: a dense novel.
7. adj. Slow to apprehend; thickheaded.

Century Dictionary and Cyclopedia

1. Having great or unusual consistency of elements or closeness of parts; closely compacted or conglomerated; compact; close; thick: as, a dense body; a dense cloud or fog; a dense panicle of flowers.
2. In zoology, closely set; separated by very small intervals: as. dense punctures, hairs, etc.
3. In photography, more or less opaque; strong in the contrast of lights and shades: said of a negative exhibiting these characteristics, and capable of giving a brilliant print, or even, if it be too dense, a harsh one, as distinguished from a weak or thin negative, the picture on which presents small contrasts, while its film is inclined to be more or less transparent, even in the lights, and the resulting print is flat. Also expressed by strong and intense.
4. Figuratively, without break or interruption; difficult to penetrate; solid and heavy: as, dense ignorance; dense wit; dense stupidity.
5. Thick-headed; obtuse; stolid; stupid; dull.
6. Synonyms Condensed, compressed.
7. n. A thicket.
8. In optics, having a large index of refraction; capable of transmitting light at a less velocity than the velocity of light-waves in vacuo.

Wiktionary

1. adj. Having relatively high density.
2. adj. Compact; crowded together.
3. adj. Thick; difficult to penetrate.
4. adj. Opaque; allowing little light to pass through.
5. adj. Obscure, or difficult to understand.
6. adj. mathematics, topology Being a subset of a topological space that approximates the space well. See Wikipedia article on dense sets for mathematical definition.
7. adj. Of a person, slow to comprehend; of low intelligence.

GNU Webster's 1913

1. adj. Having the constituent parts massed or crowded together; close; compact; thick; containing much matter in a small space; heavy; opaque.
2. adj. Stupid; gross; crass.

WordNet 3.0

1. adj. having high relative density or specific gravity
2. adj. permitting little if any light to pass through because of denseness of matter
3. adj. hard to pass through because of dense growth
4. adj. slow to learn or understand; lacking intellectual acuity