Compact; crowded together.
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.
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.
Overcrowded; congested.
Stuck.