Not AND.

The boolean function which is true unless both its arguments are true, the logical complement of and:

A NAND B = NOT (A AND B) = (NOT A) OR (NOT B)

Its truth table is:

        A | B | A NAND B
        --+---+---------
        F | F |    T
        F | T |           T
        T | F |    T
        T | T |    F

NAND, like nor, forms a complete set of boolean functions on its own since it can be used to make NOT, AND, OR and any other Boolean function:

NOT A = A NAND A

A AND B = NOT (A NAND B)

A OR B = (NOT A) NAND (NOT B)