What is the meaning of dense?

adjective

• Permitting little if any light to pass through because of denseness of matter.
  • usage: "dense smoke"; "heavy fog"; "impenetrable gloom"
  • synonyms: heavy, impenetrable
• Hard to pass through because of dense growth.
  • usage: "dense vegetation"; "thick woods"
  • synonyms: thick
• Having high relative density or specific gravity.
  • usage: "dense as lead"
• Slow to learn or understand; lacking intellectual acuity.
  • usage: "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- Thackeray; "dumb officials make some really dumb decisions"; "he was either normally stupid or being deliberately obtuse"; "worked with the slow students"
  • synonyms: dim, dull, dumb, obtuse, slow