That is a very good point. You really can't start calling it a pin until it... pins!
Maggie, it actually depends on the dojo or the instructor.
Some will talk about "applying the pin" when the hand/wrist position is placed at the start of the technique.
I tend to talk about that as applying a "lock" (working on locking up through the distal joint up through the limb and to uke's center) and reserve the "pin" for the final control.
Semantics...will get us every time!