dense

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

• adjective Having relatively high density.
• adjective Crowded closely together; compact.
• adjective Hard to penetrate; thick.
• adjective Permitting little light to pass through, because of compactness of matter.
• adjective Opaque, with good contrast between light and dark areas. Used of a photographic negative.
• adjective Difficult to understand because of complexity or obscurity.
• adjective Slow to apprehend; thickheaded.

from The Century Dictionary.

• In optics, having a large index of refraction; capable of transmitting light at a less velocity than the velocity of light-waves in vacuo.
• 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.
• In zoology, closely set; separated by very small intervals: as. dense punctures, hairs, etc.
• 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.
• Figuratively, without break or interruption; difficult to penetrate; solid and heavy: as, dense ignorance; dense wit; dense stupidity.
• Thick-headed; obtuse; stolid; stupid; dull.
• Synonyms Condensed, compressed.
• noun A thicket.

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

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

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

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

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

• adjective having high relative density or specific gravity
• adjective permitting little if any light to pass through because of denseness of matter
• adjective hard to pass through because of dense growth
• adjective slow to learn or understand; lacking intellectual acuity