I do find that ‘open world’ is used interchangeably with ‘non-linear’. I think this is a problem because they’re quite different.

Open world needs some kind of sandboxing mechanic. Whether it is building something, changing the environment, or whatever. It doesn’t have to be base building but it is the common go-to. There is usually less ‘progression’ and more isolated ‘accomplishments’ which may or may not have tangible rewards impacting game mechanics. Open worlds don’t even have to have ‘endings’.

Non linear gameplay needs things like optional and auxiliary components but also missable/altered content/choices matter, different paths/routes, and/or multiple endings affecting a core/linear game progression. Non linear games tend to ‘open up’ and ‘close off’ with lineated progression.