What are non-periodic heals in world of warcraft?

The distinction is in if the healing effect is a 'tick' or directly attributable to the spell. Whenever someone is healed, ask this question:

Does the spell that caused the heal to happen cast in the same frame/game logic tick as the heal itself?

Iff yes, then this is a direct heal.

Iff no, and there are multiple instances of time in which heals happen, it is periodic. This includes both DoT and channelled effects.

Counts as periodic

• Area effects with 'ticks'. (Healing Rain, shaman).
• DoT spells (Rejuvenation, druid)
• Drain life (Warlock)

Not actually periodic

• Area heals (Healing Circle, priest)
• Direct heals (Chain heal, shaman).
• Combined DoT/Direct spells (Riptide, shaman)¹.
• Healthstone (Warlock)

1: Only the initial cast will remove a Grievous stack, not every tick.

Interesting cases

For some spells/effects, it's unclear of whether they would count, and likely up to the developer. As it's unlikely every corner case was covered, there may be some surprising inconsistent details. Consider effects like:

• Living seed. (Adds a separate instance of heal not from a spell.)
• Priest' Trail of light (again, separate instance)
• Cloudburst totem
• (etc.)

It's likely up to the specific code implementation of each spell whether or not Grievous would count them as periodic or not. Someone would have to make a giant list of spells and test each one if they count.

It's quite likely most of these would not count as periodic with a few exceptions.