Answer:
Swing voters are people who have not made up their minds at the start of the campaign.
Explanation:
A swing vote is that which is not decided and could go, in fact, in various ways and pick one among many candidates (if there are many) or one between two. This type of voter is not affiliated with any particular political party. They could vote for Trump or Bernie Sanders, or could not vote at all. This is a percent of voters that can't be really accounted for as there aren't many ways to catalogue them or to really influence them.