If alkeneaddition involves adding non-symmetrical reagents to a non-symmetrical alkene, then two isomeric products (constitutional isomers) can be obtained. For example, the reaction of HCl with propene gives 1-chloropropane and 2-chloropropane.
Normally, 2-chloropropane is the major product, also called the Markovnikov product (because the electrophilic hydrogen ion is added to the side of the alkene with more H's). Why is this? This result is related to the stability of the carbocationintermediate within the rate-determining step of the reaction mechanism.
Since one product is favored over the other, the reaction is said to be regioselective.
If 2-chloropropane were the only product then the reaction is said to be regiospecific.