A red team refers to an independent group that challenges an organization to improve its security effectiveness by assuming an adversarial role or point of view. 

The red team term is derived from war games, where red represents the aggressor whose job it is to test the capabilities of those on defense (the blue team). In pen testing, red team attackers have no restrictions imposed on them with regard to scope or time. This is different from typical penetration tests that often set limits around both of these parameters.

Operating more open-endedly, red team tests provide the most accurate simulation of a real-world attack scenario.