It stands for time-weighted average price (TWAP) at midpoint of national best bid and best offer (NBBO); it is the midpoint of the time-weighted average NBB (national best bid) and the time-weighted average NBO (national best offer) over a period of time.
It is an NBBO midpoint that represents the midprice of the TWAP NBB and the TWAP NBO:
TWAP NBBO = (TWAP NBB + TWAP NBO)/ 2
The resulting price will be rounded to the nearest tick size.
For more on TWAP NBBO midpoint, see: TWAP NBBO midpoint: an example (calculation).
