People shorten it to NTP. The contract often counts the days to completion from the date in the notice.
Example: a contract allows a set number of calendar days from the notice to proceed. Liquidated damages apply after that.
For an estimator, the gap between bid and notice to proceed matters. Supplier quotes can expire and wages can rise in that gap. Ask the general contractor for the expected start date. Do not order material on a verbal start if your subcontract requires written notice.