A One-word term for Magical Resistance is a bit hard. Most RPG and MMO players are fairly used to composite terms for something like that, the two most common being Magical Resistance and Magical Armor.
Those are frequently abbreviated to MR and MA, respectively, which is what commonly appears on character sheets. M. Resistance and M. Armor are also pretty common forms of those terms, and most people who are used to the hobby will immediately get what they mean - and if you couple them with a shield with some sort of fireball inside of it for an icon, even better.
However, if you really, really need to use a single word it, you have some choices.
- Spirit - This is a somewhat less used term that sometimes means magical defenses and sometimes means regeneration rates. Other games may add different meanings to it, so it heavily depends on the context of use. When paired with Intelligence, Spirit is normally used for the defenses while Intelligence is used for the attack aspect of it.
- Willpower or Will - This is a common word for RPGs that more often than not refers to the ability of the character to stave off magical effects that affect the mind. A few games also generalize it to mean "magical defenses".
- Tenacity - A barely used term that when used normally refers to the ability to reduce harmful effects. Also, a really cool word.
- Resilience - Old-time WoW players will remember this one. On the WoW context, it was some sort of catch-all PVP defense, but it can be used safely to mean magical armors of sorts. Resilience is the term I use on my own RPG System for my "magical defense stuff".
Despite my suggestions, I really, really recommend you to use some sort of M.Armor or something like that. It is way easier to associate M.Armor to "Magical defenses" than it is for any other single-term word!