Let's say we have a multiclass Rogue (Assassin) + Sorcerer named Adam. Adam manages to attack a Surprised Target, and chooses to cast Fireball instead of making a Ranged Attack.
Assassinate is worded as the Following:
Assassinate: Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.
A Spell that forces a Saving Throw is obviously not an Attack, so you wouldn't get "Advantage" on it (since you as the Assassin aren't rolling). However, Assassinate doesn't explicitly state that it relies on an attack; all it says is that hits on a Surprised Target are Critical Hits.
When someone is forced to make a Saving Throw against a spell like Fireball, is that considered a "Hit" for the purpose of being doubled?
Using common definitions, it would be, since a Hit is, according to Merriam Webster:
Hit: to strike or be struck by (someone or something) forcefully
In this case, the enemy would be "struck" by the Fireball.
Or is there RAW that states that only Attacks can "Hit", or redefines the term "Hit" so that common definitions do not apply?