I mean, I am just asserting this definition here.
I'm not the King of the Dictionary.
(Although, apparently, Lords Merriam and Webster do seem to agree with me. I did not actually know this existed in 'the dictionary' prior to writing that.)
Its a slang term, used mainly by Zoomers and younger, and nobody actually seems to be able to consistently, precisely, define it.
As best I understand, its a relationship that is best defined by at least one person in it being fundamentally confused by what the 'relationship' even is.
Further attempt to explain why I define it this way
Back in the last millenium or thereabouts... we'd just call a 'situationship' something like "they're not a good fit for each other" or "x thought it was / wanted it to be serious, y didn't, thought it was totally casual" or "x (and y) doesn't seem to know what they want from y (or eachother)".
We didn't really have a specific word for it.
Like, I've dated girls who slept with me on the first date, just started referring to us as boyfriend/girlfriend by date 2.
I've also dated girls where we did not have sex untill months in, and they literally requested that I ... basically formally propose to them that we were now boyfriend and girlfriend, even though we weren't dating anyone else, and it had been like 3 months... it was very important to her that I essentially formally propose our 'we are officially dating' status.
Different people all literally play by different rules.
And you have to 1] actually know what they are for yourself and 2] tell the other person what they are.
... It is totally normal to not know what you want out of a relationship, to not know what kind of person you want to be in a ... more than friendship relationship with.
Its also pretty normal to feel socially pressured to just even have or not have ... any, or some specific kind of a relationship.
You kinda just have to learn by trial and error, learn both who yourself is as a person, and what other kind of person you are looking for.
Thats... basically what maturing, growing, as yourself, into yourself, as a player in the game of society... is.