All 3 Uses

laboratory
in
Hopeless, by Colleen Hoover
(Auto-generated)

—Never use the restroom next to the science lab.†
p. 16.2

lab = a workplace where people do scientific or medical research, or produce drugs or chemicals (also used figuratively) OR (as an adjective) related to such a place
The classes aren't so bad, but I had to (out of pure necessity and unfamiliarity) use the restroom next to the science lab, and although I survived, I'll be scarred for life.†
p. 17.4 *
I just finished it during computer lab.†
p. 20.8

Definitions:

(1) (laboratory)
a workplace where people do scientific or medical research, or produce drugs or chemicals

or (as an adjective): related to such a place
(2) (meaning too rare to warrant focus)
Much more rarely, lab can be used as an abbreviated way of saying Labrador retriever. It is also a rare last name and may indicate someone's initials.