Alan Kay coined the term "object-oriented" for software architecture in the late '60’s. He offers a good explanation of it: http://programmers.stackexchange.com/a/58732

“OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things.” — Alan Kay

It's easy to assume only languages with Class and Object abstractions are "object-oriented", but the "orientation" is really more about your mental model of the system as differentiated objects that communicate via messages.