Semistable

When a Prime $l$ divides the Discriminant of a Elliptic Curve $E$, two or all three roots of $E$ become congruent mod $l$. An Elliptic Curve is semistable if, for all such Primes $l$, only two roots become Congruent mod $l$ (with more complicated definitions for $p=2$ or 3).

See also Discriminant (Elliptic Curve), Elliptic Curve