Ethical non-monogamy is the umbrella term for relationship structures involving multiple people conducted with everyone's knowledge and consent. The adjective "ethical" distinguishes these arrangements from cheating, from non-monogamy conducted without the knowledge or consent of everyone involved.
That distinction is the minimum bar. But in practice the word "ethical" gets used in ways that suggest it means more, and sometimes in ways that suggest people think meeting the minimum bar is sufficient for any arrangement to be beyond criticism. Neither is quite right.
What consent actually requires
The defining feature of ENM is consent. Everyone involved knows what's happening and has agreed to it. But consent has conditions, it has to be informed, it has to be genuine, and it has to be ongoing.
Informed consent means people know what they're agreeing to. If a partner agrees to an open relationship without understanding what that means in practice, the time, the emotional labour, the possibility of their partner falling in love with someone else, the consent is technically present but not fully informed.
Genuine consent means the agreement wasn't made under pressure. "I agreed because the alternative was losing the relationship" is a real form of consent, but it's less robust than "I agreed because I actually wanted this." Both can be functional; the first tends to produce more fragile arrangements.
Ongoing consent means consent can be withdrawn or renegotiated. An agreement made two years ago doesn't necessarily describe what someone consents to today. Treating prior consent as permanent, as something that, once given, can't be reconsidered, misunderstands what consent is.
Consent is necessary but not sufficient
An ENM arrangement can have full, genuine, informed consent from all parties and still involve behaviour that's worth critiquing. Consent doesn't make everything that happens within a relationship structure automatically okay.
The most common example: the one penis policy. Two people in a relationship may consent to an arrangement where she can have connections with women but not with men, while he has no restrictions. Both people have consented; the arrangement is "ethical" in the minimum sense. The criticism, that the arrangement is structurally sexist, that it treats her sexuality as less threatening than his, that it reflects and reinforces an unequal dynamic, doesn't disappear because both people agreed to it.
Similarly: a relationship where one person has substantially more freedom, substantially less accountability, and substantially more influence over the structure than the other can have full consent from both parties and still involve an imbalance worth examining. Consent to an arrangement doesn't mean the arrangement is beyond assessment.
The ethical floor vs the ethical ceiling
Consent is the ethical floor, the minimum below which non-monogamy isn't ENM at all, just cheating. But the ethical ceiling, what genuinely good non-monogamy looks like, is higher than the floor.
The ceiling involves things like: treating all partners' needs as genuinely mattering, not just formally; being honest about your situation even when honesty is inconvenient; not using CNM frameworks to justify controlling or manipulative behaviour; taking the wellbeing of people you're in relationships with seriously even when their interests conflict with your preferences.
The term "ethical non-monogamy" is sometimes used as though clearing the floor is the same as reaching the ceiling. This is probably a consequence of the term's origin, it was coined to distinguish consensual arrangements from cheating, and clearing that bar was the point. But it's created a rhetorical situation where "ethical" does less work than the word implies.
What makes non-monogamy worth doing well
The more interesting question isn't "is this technically ethical?" but "is this working well for everyone involved, and if not, what would make it work better?"
Non-monogamy done well involves genuine care for everyone in the arrangement, not just formal compliance with consent standards. It involves honesty that goes beyond technical disclosure. It involves taking seriously the interests of people in less structurally powerful positions, secondaries, new partners, people without veto rights, rather than treating them as irrelevant as long as they've signed on.
The word "ethical" in ethical non-monogamy is an aspiration as much as a description. The aspiration is worth taking seriously.