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The term free streaming is used to describe particles that are traveling with essentially no absorption or scattering, examples in the current universe being neutrinos, CMB photons, and perhaps dark matter particles. A type of particle whose mean free path travel time is greater than Hubble time would definitely qualify.
The term is also used to describe such free travel of a particle over some significant distance. For example, photons from the Sun reaching Earth can be said to have free streamed from the photosphere to Earth's surface. Before that, they were not free streaming since within the Sun, they were constantly absorbed and scattered. Neutrinos formed in the Sun's core could be said to free stream out of the Sun, and those that reach Earth free stream through it.
In cosmology, the conditions under which particles free stream is of interest: in some cases, regarding some point in the history of the universe when their free streaming became common, e.g., relic neutrinos, and CMB photons.
The other area of cosmological interest is in relation to structure formation from dark matter halos (i.e., in halo models): under what conditions do particles of interest escape such halos due to a tendency to free-stream out of them, something that depends upon particle speed (which often depends upon the particle mass) as well as the mass and size of the halo. In this area, the concepts free stream scale and free stream length are used to characterize the conditions where particles will escape or not. Such escape means mass loss for the halo, tending to smooth the universe's density, i.e., reduce anisotropies.