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).