Atoms that are too close together in an organic molecule as to prevent the reaction by SN2 back-side attack. mostly refers to technical organic chemistry. Slang: can be used to refer to actually doing well on a chem test and diverting a back-side attack.
"did you get back-side attacked by that orgo test?"
"no, luckily i had enough steric hinderance to prevent an SN2 reaction"