What's the difference between a hookup, ONS, and Friends with benefits?

Can anyone define these three for me? I mean I know a ONS is a one time fuck and that's it. Friends with benefits is when it's a consistent fuck buddy. Someone you fuck from time to time. So how does hookups play into this?

I ask this because I've seen some people say they've had hookups but no friends with benefits or ONS. How?
What's the difference between a hookup, ONS, and Friends with benefits?
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