The NOR Gate

Definition, as a combination. A NOR gate (short for 'NOT-OR') is functionally an OR gate (Section 9.26) whose single output is immediately passed through a NOT gate (Section 9.28) -- its output is, at every instant, the exact complement of what a plain OR gate, fed the same two inputs, would produce.

Symbol. The NOR-gate symbol (shown in the accompanying figure) is drawn EXACTLY like the OR-gate symbol of Section 9.26 -- curved input edge, single pointed tip -- but with a small inversion bubble added at the pointed tip, the same bubble convention used throughout Sections 9.28-9.29: the bubble alone converts the OR symbol into the NOR symbol.

Truth table and Boolean equation. For two inputs $A$ and $B$, the NOR gate's truth table (shown alongside) is the row-for-row COMPLEMENT of the OR gate's own truth table (Section 9.26): output $Y=1$ only in the single row where $A=0$ AND $B=0$, and $Y=0$ in every other combination. Its Boolean equation is written

the bar drawn over the WHOLE sum $A+B$, showing the inversion is applied to the combined OR result, not to $A$ or $B$ individually. …