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(US military jargon) 1. Originally, a team whose purpose is to penetrate security, and thus test security measures. These people are paid professionals who do hacker-type tricks, e.g. leave cardboard signs saying "bomb" in critical defence installations, hand-lettered notes saying "Your codebooks have been stolen" (they usually haven't been) inside safes, etc. After a successful penetration, some high-ranking security type shows up the next morning for a "security review" and finds the sign, note, etc. and all hell breaks loose. Serious successes of tiger teams sometimes lead to early retirement for base commanders and security officers (see the patch entry for an example).
2. Recently, and more generally, any official inspection team or special firefighting group called in to look at a problem.
A subset of tiger teams are professional crackers, testing the security of military computer installations by attempting remote attacks via networks or supposedly "secure" communication channels. Some of their escapades, if declassified, would probably rank among the greatest hacks of all times. The term has been adopted in commercial computer-security circles in this more specific sense.