1. permitting little if any light to pass through because of denseness of matter

- dense smoke

Similar word(s): thick, heavy, impenetrable

2. hard to pass through because of dense growth

- dense vegetation

Similar word(s): impenetrable, thick

3. having high relative density or specific gravity

- dense as lead

Similar word(s): heavy

4. slow to learn or understand; lacking intellectual acuity

- so dense he never understands anything I say to him

Similar word(s): stupid, dim, dull, dumb, obtuse, slow