Opaque; allowing little light to pass through.
Thick; difficult to penetrate.
Obscure, or difficult to understand.
Slow to comprehend; of low intelligence.
Having relatively high density.
Compact; crowded together.
Being a subset of a topological space that approximates the space well. See the Wikipedia article on dense sets for a mathematical definition.
A thicket.
The situation where an event is so well attended, or public transportation so full, that all of the seats in the venue are occupied, leaving only the ground or the floor for other people.