Picture lock
The point at which the edit stops changing, so finishing work can safely begin.
Picture lock is the agreed moment when no further changes will be made to the timing or order of the cut. Everything downstream (colour grade, sound mix, graphics, subtitles) is built against a fixed timeline.
The reason it's a formal milestone rather than a vibe is cost. Moving a shot after lock invalidates work that was keyed to timecodes: graphics land in the wrong place, the mix drifts, subtitle files desynchronise. A change that costs minutes before lock can cost a day after it.
If you need one thing from a review process, it's a clear lock. Say the word out loud, put it in writing, and treat anything after it as a change request with its own cost, not as a note.
Where this comes up
Two rounds is usually right. Why projects need five, how to consolidate feedback so an editor can act on it, and what to do when a late stakeholder appears.
A naming convention, the right formats per destination, and an expiry you state up front. The unglamorous half hour that prevents months of "can you resend".