Astrochemistry - what does freeze-out mean?
Freeze-out is indeed a chemical term. It happens in the midplane of accretion disks where density is high enough to block UV ionization/chemical dissociation and limit heating from the central star. No one seems to have gotten around to writing up a layman friendly article on the subject yet, but these refs give the gist:

Protostars and planets V

Accretion Disks 1: During Star Formation

Organic Matter in Space (IAU S251) Evolution of Organic Matter

From ref 2: