In Boolean logic, logical NOR, non-disjunction, or joint denial is a truth-functional operator which produces a result that is the negation of logical or. That is, a sentence of the form (p NOR q) is true precisely when neither p nor q is true—i.e. when both p and q are false. It is logically equivalent to ${\displaystyle \neg (p\lor q)}$ and ${\displaystyle \neg p\land \neg q}$, where the symbol ${\displaystyle \neg }$ signifies logical negation, ${\displaystyle \lor }$ signifies OR, and ${\displaystyle \land }$ signifies AND.

Non-disjunction is usually denoted as ${\displaystyle \downarrow }$ or ${\displaystyle {\overline {\vee }}}$ or ${\displaystyle X}$ (prefix) or ${\displaystyle \operatorname {NOR} }$.

As with its dual, the NAND operator (also known as the Sheffer stroke—symbolized as either ${\displaystyle \uparrow }$, ${\displaystyle \mid }$ or ${\displaystyle /}$), NOR can be used by itself, without any other logical operator, to constitute a logical formal system (making NOR functionally complete).

The computer used in the spacecraft that first carried humans to the moon, the Apollo Guidance Computer, was constructed entirely using NOR gates with three inputs.

The NOR operation is a logical operation on two logical values, typically the values of two propositions, that produces a value of true if and only if both operands are false. In other words, it produces a value of false if and only if at least one operand is true.

The truth table of ${\displaystyle A\downarrow B}$ is as follows:

The logical NOR ${\displaystyle \downarrow }$ is the negation of the disjunction:

Peirce is the first to show the functional completeness of non-disjunction while he doesn't publish his result. Peirce used ${\displaystyle {\overline {\curlywedge }}}$ for non-conjunction and ${\displaystyle \curlywedge }$ for non-disjunction (in fact, what Peirce himself used is ${\displaystyle \curlywedge }$ and he didn't introduce ${\displaystyle {\overline {\curlywedge }}}$ while Peirce's editors made such disambiguated use). Peirce called ${\displaystyle \curlywedge }$ the ampheck (from Ancient Greek ἀμφήκης, amphēkēs, "cutting both ways").