Xor

"A or B, but not both"

Exclusive or (usual symbol XOR occasionally EOR), which is sometimes called exclusive disjunction, is a logical operator that results in true if one of the operands, but not both of them, is true.

+------+---+
| A, B | X |
+------+---+
| 0  0 | 0 |
| 0  1 | 1 |
| 1  0 | 1 |
| 1  1 | 0 |
+------+---+

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• Wikipedia:XOR
• http://mathworld.wolfram.com/XOR.html