The concept of fairness is being discussed related to the distribution of voting power among different actors of voting. We are using the following, intuitively natural principle of fairness: each unit of resources (shares, seats) should yield the same voting power (influence).
Voting power is not directly observable: as a proxy for it voting weights are used. Therefore, fairness is usually defined in terms of voting weights (e.g. voting weights are proportional to the results of an election).
Assuming that a principle of fair distribution of voting weights is selected, we are addressing the question of how to achieve equality of relative voting power (at least approximately) to relative voting weights.