stereoselective refers to those reactions where the product may have multiple configuration but the obtained one is one of them for ex in SN2 u may have a d 0r a l isomer but not both Stereospecific means two different isomers under identical condition give different products ex. Brimination of alkenes Regioselective:If u get high amount of a specific product after a bond breaking ex. OMDM Regiospecific:if u get only one kind of isomer with no other choice ex markonikov addition