Board Thread:The Panopticon/@comment-1293767-20151029072618/@comment-5918438-20160108024502

And I should stress that what we mean when we say "two-parter" is that "these two stories belong to the same one Doctor Who television story, which has two (named) parts". And the thing about singular TV stories with multiple parts is that they all share one director, they're all produced together as one story, and they all have a lot of the same guest cast shared between them. And they are always broadcast one after the other in the same run, with no other TV stories in between.

"Two-parter" is a production term, in the end, and is as intricately linked to production blocks as Doctor-lite is to double banking. Yes, people use Doctor-lite to mean "any story of Doctor Who which features the Doctor less than usual", but "less than usual" cannot be defined, and so the scope of "Doctor-lite" cannot be properly defined if used in that manner. Likewise, we cannot use "two-parter" as loosely as that.
 * "Instead of filming and contracting for single episodes, directors, script editors, directors of photography and some actors — along with many other individuals — are assigned to groups of episodes"

Stories which are truly multi-part, which are made by the BBC Wales Doctor Who team, are always produced in that manner. A lot of the "questionable" first parts of stories do not follow those rules.