When deciding policy, political parties only care about the people who will vote based on that policy, or could be induced to vote based on that policy. Undecided voters might as well not exist (when they stay undecided). The normal definition of majority in politics is with undecideds excluded, and they're typically only excluded by people dishonestly trying to downplay their opponents' support. In local elections, people who are eligible to vote but stay home typically outnumber people who do vote, but council seats are still given to people who win a majority, even though it's a tiny slice of the people eligible to vote who voted for them.

It's fine if you're just learning that this is the normal definition in political contexts just now, but you're going to get into a lot of arguments if you use different definitions of words to the ones the people you're communicating with are.