Isogeny of an elliptic curve

Question

Let $E$ be an elliptic curve over $\mathbb{Q}$ and $p$ be a prime. Then what does it mean by "$E$ has a $\mathbb{Q}$-isogeny of degree $p$"?

Answer 1

As Robert Auffarth said in a comment, a $\mathbb{Q}$-isogeny of degree $p$ is a non-constant morphism of elliptic curves $E\to E'$ defined over $\mathbb{Q}$, that sends zero to zero, i.e., $\mathcal{O}_E\mapsto \mathcal{O}_{E'}$, and has degree $p$. Since the degree of the map equals the size of the kernel, the degree $p$ condition means that the kernel has size exactly $p$.

As it turns out (this is hard), there are only $\mathbb{Q}$-rational isogenies of elliptic curves of degree $p$ for finitely many primes, namely $2,3,5,7,11,13,17,19,37,43,67$, and $163$.