A nesting partner is one you share a home with; an anchor partner is the steady, entangled relationship you build your life around — both name closeness by shared life rather than by rank.
For a long time polyamory borrowed its vocabulary from the office: primary partner, secondary partner, as if love came with an org chart. "Nesting" and "anchor" are the pushback. They describe what a relationship is — who you share a fridge with, who you'd call from the hospital — instead of stamping it with a rank.
The two aren't synonyms. A nesting partner is about the nest: same address, shared rent, whose turn it is to buy toilet paper. An anchor partner is about gravity: the long-entangled relationship you steer big life decisions around, whether or not you live together. Often it's the same person. Sometimes your anchor lives across town and your nesting partner is a dear housemate you're not building a mortgage with. The words let you say which is which.
The honest catch: swapping "primary" for "anchor" doesn't automatically delete hierarchy — it just stops announcing it. You can call no one primary and still quietly give one person veto over everyone else. These are descriptive tools, not moral upgrades. Used well, they let each relationship be named for what it actually is; used lazily, they're the same ranking with prettier branding.