a relationship, usually a couple, that agrees to sexual or sometimes romantic connections outside the pair, with the original relationship understood as the anchor. "open relationship" is often used loosely as a catch-all for non-monogamy, but most specifically it describes keeping one central partnership with agreed room around it. it shares polyamory's honesty and openness, and tends to differ in shape: one primary bond with space around it, rather than several relationships each allowed to matter fully.
"Open relationship" is one of the most elastic terms in this whole vocabulary. Some people use it to mean a couple who occasionally see others sexually; some use it as a soft on-ramp word before they have settled on anything more specific; some use it interchangeably with polyamory. Because it stretches so far, the honest move is usually to ask what a particular couple means by it rather than assume.
In its most specific sense, an open relationship keeps one partnership at the center and opens a defined space around it. That is the common distinction from polyamory, which is more willing to let several relationships carry full romantic weight without a built-in hierarchy, though plenty of polyamorous arrangements also have a central partnership. Both live under ethical non-monogamy; the difference is how much the shape is organized around one anchor.
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definition contributed by Tessakin