regioselectivity

When a reaction could attach a new group, or form a new bond, at more than one possible position on a molecule, and it prefers one of those positions over the others, we say the reaction is regioselective. Regio- refers to region or position; regioselectivity is simply the reaction caring about where, not just whether, something happens. Zaitsev's rule and Hofmann's rule are two famous statements of the regioselectivity of elimination.

In elimination specifically, the question is which beta hydrogen is removed — because removing the H from one neighbouring carbon versus another gives double bonds in different positions, that is, different constitutional isomers. A reaction that strongly prefers one orientation (say, always the more substituted alkene) is highly regioselective; one that gives a near-even mixture has poor regioselectivity. The same idea governs additions to alkenes (Markovnikov versus anti-Markovnikov) and aromatic substitution (ortho/para versus meta).

Keep regioselectivity distinct from stereoselectivity. Regioselectivity is about position — which carbon, which region of the molecule. Stereoselectivity is about three-dimensional arrangement — which enantiomer or which E/Z geometry — at a position already chosen. An E2 can be both regioselective (favouring the more substituted alkene) and stereoselective (favouring a particular E or Z product because of the anti-periplanar requirement), and being clear about which kind of selectivity you mean prevents a lot of confusion.