# Stress energy components as a 3 or 4 density?

I usually come across expressions for the Stress energy tensor showing them as three densities (normally over a space-like slice.
In wikipedia's article on the spin tensor, they clearly (it does not appear to be an error) both write about and express the components of the stress energy tensor as four-densities. Is the three density just an approximation then?
For example:

I usually come across this as an integral over three-space.

Orodruin
Staff Emeritus