A reaction that leads predominantly to one stereoisomer is stereoselective. Radical halogenations of diastereotopic hydrogens generate diastereoisomers in different amounts, therefore they are said to be stereoselective reactions.

 

A reaction that leads exclusively to a particular stereoisomer is said to be stereospecific. In the subject of substitutions and eliminations we will see that S_N2 is a reaction that gives only one of the possible stereoisomers, therefore it is a stereospecific reaction.

The halogenation of enantiotopic hydrogens leads to a mixture of enantiomers in equal amounts, therefore it lacks any selectivity.