What is Don't Ask, Don't Tell?

Legal Definition
"Don't ask, don't tell" (DADT) was the official United States policy on military service by gays, bisexuals, and lesbians, instituted by the Clinton Administration on February 28, 1994, when Department of Defense Directive 1304.26 issued on December 21, 1993, took effect, lasting until September 20, 2011. The policy prohibited military personnel from discriminating against or harassing closeted homosexual or bisexual service members or applicants, while barring openly gay, lesbian, or bisexual persons from military service. This relaxation of legal restrictions on service by gays and lesbians in the armed forces was mandated by United States federal law Pub.L. 103–160 (10 U.S.C. § 654), which was signed November 30, 1993. The policy prohibited people who "demonstrate a propensity or intent to engage in homosexual acts" from serving in the armed forces of the United States, because their presence "would create an unacceptable risk to the high standards of morale, good order and discipline, and unit cohesion that are the essence of military capability".

The act prohibited any homosexual or bisexual person from disclosing his or her sexual orientation or from speaking about any homosexual relationships, including marriages or other familial attributes, while serving in the United States armed forces. The act specified that service members who disclose that they are homosexual or engage in homosexual conduct should be separated (discharged) except when a service member's conduct was "for the purpose of avoiding or terminating military service" or when it "would not be in the best interest of the armed forces". Since DADT ended in 2011, persons who are openly homosexual and bisexual have been able to serve.

The "don't ask" part of the DADT policy specified that superiors should not initiate investigation of a service member's orientation without witnessing disallowed behaviors, though credible evidence of homosexual behavior could be used to initiate an investigation. Unauthorized investigations and harassment of suspected servicemen and women led to an expansion of the policy to "don't ask, don't tell, don't pursue, don't harass".

Legislation to repeal DADT was enacted in December 2010, specifying that the policy would remain in place until the President, the Secretary of Defense, and the Chairman of the Joint Chiefs of Staff certified that repeal would not harm military readiness, followed by a 60-day waiting period. A July 6, 2011, ruling from a federal appeals court barred further enforcement of the U.S. military's ban on openly gay service members. President Barack Obama, Secretary of Defense Leon Panetta, and Chairman of the Joint Chiefs of Staff Admiral Mike Mullen sent that certification to Congress on July 22, 2011, which set the end of DADT to September 20, 2011.
-- Wikipedia