Certain abbreviations are used pervasively throughout the GHC source code. A few, like "Tc", are easy to figure out. Others, like "Occ", are more challenging (in case you were wondering, none of GHC's code is  Occult, except perhaps in the Latin sense)

See also:  A similar list on the GHC Users' wiki.

* "Occ" means "Occurrence"

• However, in the context of  OccName, "occurrence" actually means "unqualified"

* "Rn" means "Renamer"

* "Rdr" means "Parser" (do not ask me why, I don't know)

* "Tc" means "TypeCheck?{ing,er}"

* "Lcl" means "Local"

* "Gbl" means "Global"

* "Loc" means "Location", as in SrcLoc?

* "Hs" means "Haskell Syntax" (generally as opposed to Core -- for example, Expr vs HsExpr?)