A Notice of Completion (NOC) is a document recorded with the county recorder by the owner (or sometimes the GC) declaring that the project is substantially complete. In states like California, recording the NOC starts a shortened lien-claim deadline window: typically 30 days for direct contractors and 60 days for everyone else, instead of the longer default. Owners use the NOC strategically to force lien claims to surface quickly so that title can be cleared for refinancing, sale, or tenant occupancy.
For contractors and suppliers, the NOC creates a deadline trap: if your right to lien depends on the longer default deadline and the owner records an NOC you missed, you may lose your lien rights without knowing. Best practice: subscribe to a recording-monitoring service in California (and similar NOC states), or check the recorder's office before the default deadline expires. The NOC is not the same as the certificate of occupancy or substantial completion; those are different documents serving different purposes.