What is the difference between hasad and the evil eye?

The two overlap and are often used interchangeably, but there is a meaningful distinction.

Hasad is envy in the heart: wishing that a blessing someone enjoys would be taken from them. It is a sin regardless of whether it produces any external effect, and Surah Al-Falaq instructs believers to seek refuge from the envier when he envies.

Al-ayn is the harm that reaches a person through the look. It is broader than hasad, because it can occur without any ill will at all. A mother admiring her own child, a friend genuinely delighted by good news, a person catching sight of something beautiful. All can transmit harm if the moment passes without the blessing being attributed to Allah.

So every envious look risks the eye, but not every eye involves envy.

The practical consequence: protection is needed against admiration as much as against malice, which is why the Sunnah emphasises saying masha Allah and asking for barakah when praising anything.