For questions about the paper or other instrument with which a voter's choices are stored. May be a paper, a colored ball, or electronic storage. Not for questions about choices which might appear. Consider referendum instead.
Wikipedia says:
A ballot is a device used to cast votes in an election, and may be a piece of paper or a small ball used in secret voting. It was originally a small ball (see blackballing) used to record decisions made by voters.
For our purposes here, we should interpret this broadly. A piece of paper, a colored ball or shell, or a row in a database should all count as ballots. This is not to say that all are of equal quality but only to say that they are or have been in use as ballots.
If you have a question about a ballot choice, consider referendum.
Consider voting-system for questions about the system of choosing the winner based on choices recorded on the ballots.
For questions about the actual process of voting, we already have a tag. We can reserve ballot for just the questions about the instrument used to store the vote. In cases of paper ballots, this may also involve discussion of casting the vote. Do people vote with a hole punch? A pencil? A marker?
A voting machine may mark up a ballot, but it is not the ballot itself. Consider electronic-voting for questions about the device. Or voting for mechanical devices.
