An agreement to be non-monogamous while deliberately keeping the details of outside relationships off the table — no questions asked, nothing told.
DADT is the arrangement where you agree your partner can be with other people, and you agree not to know about it. The deal isn't secrecy in the cheating sense — it's consented ignorance. Both people signed off on the not-knowing in advance: I won't ask what you get up to, you won't hand me the play-by-play. For some couples that's a clean, functional boundary. "I trust you, I don't want a running commentary, keep it safe" is a legitimate thing to want.
Here's where it gets argued about. A lot of polyamory folks won't file DADT under poly at all — they'll call it non-monogamy with the lights off. The friction is structural: informed consent runs on information, and DADT is built on agreeing not to have any. The person who most often gets erased is the third one — the outside partner who becomes a secret by design, easy to keep invisible and easy to treat as disposable. There's usually a power tilt too: one partner is the one out having adventures, the other is the one performing not-caring.
So it isn't automatically toxic and it isn't automatically fine. The failure mode is using DADT as a fig leaf — pre-forgiven cheating, or a way to keep someone off the books so you never have to treat them as real. The honest test is simple: does everyone in the web, including the people you've agreed not to talk about, actually know the shape of the thing and want to be in it? If the silence protects everyone's comfort, it can work. If it only protects one person from accountability, it's just a nicer word for a lie.