Selectivity in organic reactions
Yours is a common source of confusion. Reactions are can be selective but they are not specific. What's specific is a mechanism for the reaction. If the reaction only proceeds via one mechanism, then it is extremely selective.


Stereospecific. The mechanism provides a specific stereochemical outcome or proceeds from a specific stereochemical configuration. Examples:

a. Pd-catalyzed hydrogenation and hydroboration proceed via syn addition. These mechanisms are stereospecific because they indicate always adding to the same side of an olefin.

b. $\mathrm{E}_{2}$ elimination processes require that the hydrogen and leaving group are anti-periplanar. This mechanism is stereospecific.

c. Acid-catalyzed hydration proceeds via a planar carbocation intermediate where previous stereochemistry might be lost. This mechanism is not stereospecific.
Stereoselective. The reaction provides more of a product with a specific stereochemistry than any of the other possibilities.

a. Allylic alcohol directed epoxidation reactions frequently yield different amounts of the two product epoxides that result from reacting on the different faces of the olefin. This reaction is selective.

b. Asymmetric reactions that rely on pathways through diastereomeric transitions states are stereoselective.

c. Acid-catalyzed hydration could be stereoselective because the molecule can have other stereocenters that provide diastereoselectivity, even though the base mechanism disregards stereochemistry.
Regiospecific. The mechanism indicates a certain connectivity that should be obtained in the product.

a. Exo- versus endo- intramolecular reactions have regiospecific mechanisms.

b. A bit of a stretch, but carbonyl addition happens at the carbon. The mechanism is regiospecific.
Regioselective. The reaction favors one connectivity pattern over the other.

a. Hydrobromination favors the Markovnikov product, while hydrobromination in the presence of peroxides favor the anti-Markovnikov product.

b. In intramolecular ring closing reactions, 5-exo-tet products are preferred over 6-endo-tet products.

c. Carbonyl addition happens at the carbon. The only mechanism in operation is the one mentioned above, so the reactions are regioselective.


Importantly, reactions should not be stereospecific or regiospecific, even if they give only one product. This is a concrete case of a reaction proceeding exclusively (or almost exclusively) via a single mechanism that is stereospecific or regiospecific, respectively. The reaction itself is, respectively, stereoselective or regioselective.