A committed couple who agree that one or both partners can have sexual — and sometimes romantic — connections with other people, out in the open.
An open relationship isn't defined by the sex with other people. It's defined by the agreement. Two people sit down, decide their couple isn't a closed box, and draw the terms together. The moment everyone knows and says yes, it stops being cheating and becomes a structure. That single word — agreement — is the whole difference between an open relationship and an affair, and it's the part people outside tend to miss.
In practice "open" is the loose, roomy end of consensual non-monogamy. Usually the couple stays each other's anchor — the primary bond — while outside connections lean more physical than romantic. That's the rough contrast with polyamory, where the point is being able to love more than one person at once. But nobody polices these labels, and the rules live in the couple: some do threesomes only, some go don't-ask-don't-tell, some allow full separate relationships. There's no certified version. There's the one you negotiated, and whether you actually stick to it.
Here's the part the wellness crowd skips: opening a relationship is not a repair kit. It doesn't fix a couple that's already leaking, and it isn't easier than monogamy — it's a different job. It runs on blunt conversations, jealousy you have to actually sit with, and, on the good days, compersion: getting a genuine kick out of your partner's pleasure with someone else. Funny origin footnote — the couple who coined the phrase never meant the sex at all. They meant honesty. The culture took the word and ran the other way.