What is the definition of plate tectonics?

The process in which the large plates that form Earth's crust are constantly moving together or apart.

The definition from what I know because I remember it from biology I think is "a theory explaining the structure of the earth's crust and some other associated phenomena as resulting from the interaction of a rigid lithospheric plates that move slowly over the underlying mantle." I'll put the quotations for you to copy what I said so it doesn't sound like you got it off me and I hopefully spelt lithospheric right