What do ITerm, BTerm, MTerm (and so on) stand for? Like, what's the meaning of the letter?

Instance Terminal Block terminal? Master terminal

Makes sense, thanks

@Arman Avetisyan is correct. Instance terms are the logical terminals to an instance (think A and Z terminals for an inverter). Block terms are the logical terminals to the inputs/outputs of your design. Master terms are the logical terminals for library cells. ITerms often reference MTerms because there's no point in duplicating common data across every instance.