Hard and soft magic
Also said as Sanderson's First Law, Magic systems
Hard magic has clear rules the audience knows. Soft magic stays mysterious. Brandon Sanderson's First Law: an author's ability to solve conflict with magic is directly proportional to how well the reader understands it.
If players know the rules, a clever solution is theirs to anticipate and it feels earned. If they do not, the same solution feels like the author cheating. Soft magic is fine, as long as it creates problems instead of solving them.
Say it
“The player must be able to solve the final puzzle with the rules we have taught. State the spell's three limits in the first hour, then build the finale so the third limit is the answer.”
Where you’ve seen it
Divinity: Original Sin 2 has very hard magic: fire ignites oil, rain makes water, and lightning shocks anyone standing in that water. Once you have seen those rules, you can plan combos the designers never specifically wrote.
Not to be confused with science fiction versus fantasy
This has nothing to do with genre or how serious the magic looks. It is only about how much the audience can predict.