Friends with benefits has a clear, if casual, understanding — sex without romance, by agreement. A situationship's hallmark is that the terms are deliberately or accidentally left undefined, often with real feelings involved.
Side by side
Aspect
Situationship
Friends with benefits
Terms
Undefined
Agreed (casual)
Romance
Often present
Usually off the table
The label
Avoided
Clear enough
Tension
'What are we?'
'We both know what this is'
What each one means
Situationship
A situationship is a romantic or sexual relationship that's been left undefined — you're involved, but without any agreed commitment, exclusivity, or label. The absence of definition is the whole point.