what is the "density" exactly means?

[0lin]

The cat, 2nd density creature, have a 2nd density body in the our space/time and is exist in 3 dimension space.

source, reference, or logic behind this ?

Law of One, Book I, Session 10
“Questioner: Have any of the Maldek entities transformed since then? Are
they still second-density now or are some of them third-density?
Ra: I am Ra. The consciousness of these entities has always been third density.The alleviation mechanism was designed by the placement of this consciousness in second-dimensional physical chemical complexes which are
not able to be dextrous or manipulative to the extent which is appropriate
to the working of the third-density distortions of the mind complex.
...
Questioner: Have any of the entities moved on now, made a graduation at
the end of a cycle and made the transition from second-density bodies to
third-density bodies?
A: I am Ra. Many of these entities were able to remove the accumulation of
what you call karma, thus being able to accept a third-density cycle within a third-density body. Most of those beings so succeeding have incarnated
elsewhere in the creation for the succeeding cycle in third density. As this
planet reached third density some few of these entities became able to join
the vibration of this sphere in third-density form. There remain a few who
have not yet alleviated through the mind/body/spirit coordination of
distortions the previous action taken by them. Therefore, they remain.”

See?It imply the body of monkey is a second-density body because their consciousness is in the second-dimensional physical chemical complexes. In this part, the questioner mixed up the concept of "density" and "dimension". Our body must be a third density one and our consciousness is placed in the third-dimensional physical chemical complexes. So the body of an 4th density entity must have a 4-dimensional consciousness.