CTV is the device connected to a TV or a smart TV capable of streaming.
OTT is content being delivered over and around a Cable Company or broadcaster.
While CTV and OTT are different many use them interchangeably. When I talk with Clients I always say CTV. If we discuss OTT, I always identify, this means Impressions On Televisions.
Keep in mind the Internet Advertising Bureau (IAB) and other distinguished leaders in the CTV | OTT space official definition for OTT streaming video focuses on the big screen televisions.
LAWonCTV.com, it is all in the name. Legal Advertising Works On CTV | OTT