It is usually called an electoral quotient.
The population of eligible voters is called the electorate.
There's no exact opposite for electorate, but the electorate is a body politic of registered voters, so at a stretch you could use 'non-voters' as an antonym.
Electorate.
It is the electorate.
Non-voters
The state electorate are the registered voters of that state.
Electorate(suffrage)
The people able to vote in an election are called the electorate.
one
The people able to vote in an election are called the electorate.
The answer will depend on which country's electorate the question is about.
enroll or enlist electorate