For questions involving selectivity as to where a functional group reacts.
As defined in the IUPAC Gold Book (DOI: 10.1351/goldbook.R05243):
A regioselective reaction is one in which one direction of bond making or breaking occurs preferentially over all other possible directions.
For example, the addition of hydrogen bromide to an alkene occurs with the C–Br bond preferentially forming at the more substituted carbon (Markovnikov's rule):
Regioselectivity may also be considered as selectivity in which one part of a functional group preferentially reacts over another. By appropriate choice of reagents or conditions, this may be controlled: