https://ippsr.msu.edu/sites/default/files/redistricting/redistrictingreportv2.pdf (about gerrymandering)
There are two alternative ideas as to what is “fair.” One notion of fairness is an idea of symmetry:
each party must be equally able to translate statewide vote share into seats. For instance, if two
parties each net exactly half the votes, symmetry requires that they each are awarded half the
seats. Despite its intuitive appeal, the Supreme Court of the United States has ruled that this idea
of fairness as symmetry is “based on a norm that does not exist in our electoral system.”
The Supreme Court of Pennsylvania proposed a different notion of fairness: the seat outcome is “neutral” if it is similar to the outcome we would expect if the electoral institutions were designed without considering partisan considerations. A redistricting map is “fair” under this second notion if it leads to neutral seat outcomes.