The TUC is running Fair Pay Fortnight.
Based on my recent experience of using the word 'fair' I am sure they will be inundated by pedants (or logical positivists, if we want to be more philosophically precise) demanding a definition of the word 'fair' or that they desist from their campaign.
My answer to the demand is twofold. First, fair is what the user makes clear they intend it to be in the context in which they use it. Second, that believe on the user's part is appropriate if they make clear what their intention is.
Of course that means the definition of what is fair is contextual and subjective. That has always been the case: that is why we disagree on whatbfairness is, so often. Disagreement on context and its subjective interpretation then becomes a fair basis for debate. But to deny this subjectivity and to so seek to suppress debate on issues of social justice - which is what any debate on fairness is ultimately about - is actuallly a disguised but quite deliberate act to perpetuate injustice. The decision to pursue such an act of oppression is, of course, itself subjective - and so in turn, self referentially, a definition of unfairness. But is it is an unfairness that some are willing to use, far too often.