A thicket.
"The hikers struggled through the dense brambles that blocked their path."
permitting little if any light to pass through because of denseness of matter
"dense smoke"
"heavy fog"
"impenetrable gloom"
slow to learn or understand; lacking intellectual acuity
"so dense he never understands anything I say to him"
"never met anyone quite so dim"
"although dull at classical learning, at mathematics he was uncommonly quick"
"dumb officials make some really dumb decisions"
"he was either normally stupid or being deliberately obtuse"
"worked with the slow students"
Having relatively high density.
"The ancient forest was so dense that sunlight barely reached the ground."
In plain English: Dense means something is packed tightly with very little empty space between its parts.
"The crowd was so dense that I could not see through them."
The word "dense" comes from the Middle French term for thick or crowded, which was borrowed directly from Latin. It ultimately traces its roots to a Proto-Indo-European root meaning "thick," linking it to similar words in Ancient Greek.