Does the Sentinel reaction melee attack count as an opportunity attack for the 1st part?

@JeremyECrawford Does the reaction melee attack granted by the 3rd part of Sentinel count as an opportunity attack for the 1st part? Speed 0

— Jonathan Elsas (@JonathanElsas) May 12, 2017

An attack counts as an opportunity attack only if it's called an opportunity attack. #DnD https://t.co/uOYmJCR28q

— Jeremy Crawford (@JeremyECrawford) May 12, 2017