1) The Don't Ask Don't Tell policy in the US military, in which homosexuality was to be overlooked as long as no one asked about it and it wasn't obvious or wasn't revealed in any manner. It was intended to give gay members of the military some freedom from persecution, but has, in practice, forced gay people into the closet when revealing their relationship or sexual orientation is cause for punishment.
2) A relationship structure in which a person who is partnered is permitted to have additional sexual or romantic relationships on the condition that his or her partner does not know anything about those additional relationships and does not meet any of those other people.
Commentary: Many people in the polyamorous community frown on DADT relationships, and choose not to become involved in such relationships. There are many dangers in such relationships, including: the idea that a person who claims to be involved in such a relationship may simply be cheating (as the relationship often provides no mechanism by which that person's partner may be contacted to confirm that the relationship permits other relationships); the fact that many people choose DADT relationships as a way of avoiding and not dealing with emotional issues such as jealousy; and the fact that DADT relationships are built on a foundation of lack of communication within the existing relationship.