in wow what is a non periodic spell?

Answer 1

A normal spell cast; ie. not a DOT (damage over time)

So basically spells like Fireball or Shadowbolt or Starfire.

Answer 2

A periodic spell is one that happens over time. E.g. a HoT or DoT (Lifebloom or Incinerate respectively, for example) whereas a non periodic spell would be just a 'regular' spell such as Fireball, Frostbolt or Flash of Light