Can lab be used not as a shorthand or abbreviation for laboratory? There are many courses titled such as
Chemistry Lab and
Physics Lab and I somewhat feel like
lab is more like a complete word.
Lab can be considered a proper word in its own right, fully substitutable in all situations. It has similar status to other shortened words like tarp and auto.
As contrasting example of the shortening process, fridge is pretty close to being an acceptable full replacement for refrigerator, though for now it is still considered slightly informal. (If you buy a fridge, the owner's manual will still call it a refrigerator.)
With various possible meanings, the abbreviation Lab is also used for Laboratory. Abbreviations.com has an entry for that. Also, note that there are other meanings for what Lab is used.
Said that, it can be used for Physics/Chemistry Lab. You have to change your feelings, lab is not a complete word but an abbreviation!
I too wondered this when heard about cath-lab