Scale invariance can refer to one of two criteria: a cardinal voting method criterion and an ordinal one.
The cardinal method criterion is:
Multiplying every ballot's score of every candidate by a constant should not change the outcome.
with a stronger variant being
Multiplying one or more ballot's score of every candidate by a constant should not change the outcome.
The ordinal method criterion is also called the homogeneity criterion. It is:
For every way of ranking the candidates, multiplying the number of voters who express this preference by a constant should not change the outcome.
These criteria represent a desideratum that the method should not rely on absolute numbers when selecting a winner, just on the candidates' or factions' relative support.
This page is a stub - please add to it.