What's the difference between open relationships and polyamory?
An open relationship typically centres a primary pair and allows outside sexual — sometimes romantic — connections within negotiated terms. Polyamory centres multiple loving partnerships; the primary-pair-with-exceptions shape is not the structure. The dividing line is whether 'partner' as a role is reserved for the central pair.
The two terms overlap. Both are configurations of ethical non-monogamy. The most useful way to distinguish them is by what counts as a partner.
In a typical open relationship, the partners are the central pair. Outside connections may be ongoing, may involve real feelings, but they are not partners in the same sense — they are recognised as outside connections that the relationship has chosen to permit. The structure is shaped around protecting the primary partnership; the language reserves the partner role for it.
In polyamory, partner is a category that can be plural. Multiple deeply-committed romantic relationships are the structure, not an exception to it. The partners across a polycule may be in hierarchical or non-hierarchical arrangements with each other, but they are not framed as 'the real partner plus some outside connections.'
Open relationships often have a more sexual centre of gravity than polyamory; the negotiated exceptions tend to be about whom you can sleep with rather than whom you can fall in love with. Polyamory is centred on the romantic-emotional dimension; multiple deep romantic attachments are explicitly the point, not the exception.
Many couples open from a monogamous starting state and pass through 'open' as a structure for a year or two before realising that what they have become is more polyamorous than the open-relationship framing accommodates. Others stay in an open configuration long-term, preferring the more legible central-pair structure. Neither path is more correct; the labels matter to the extent that they help the practitioners describe what they actually have.