What is the meaning of "density?

How close together units of [whatever thing] are.  Population density is usually expessed as people per square mile.  The individual whiskers of one man's beard may be more dense than those of another man.   A cubic unit (foot or mm) of glass has molecues packed more densely that a unit of styrofoam.  Thus the glass is heavier.

how dense something is

like heavy or the size or something

so you could say an object is dense

or you could also say a person is dense (telling them they’re stupid)

mass per unit volume..

Density is the degree of compactness of a substance.

amount of material per unit space, in basic words.

A dense object has a high weight (high mass) for its size (volume).  Dense writing has a lot of information in a small number of words.

Among other possibilities, thick and crowded are two of the most common.

Thickness of something. 

thickness                                    .

Density

Mass divided by the given volume 

D=M/V 

DV=M 

M=D/V 

"Density is fun." 

"Density is important when finding the mass"