The third benefit of the Sentinel feat is described as follows (PHB, p. 169):
- When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
Does the third benefit of Sentinel count as an opportunity attack for the purpose of reducing their speed to 0 (per the first benefit of the feat)?
For reference, the description of the first benefit of Sentinel says:
- When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.