What is Vote Trading?

Legal Definition
Vote trading is the practice of voting for or against another person's bill, position on a more general issue, or favored candidate in exchange for the other person's vote for or against a position, proposal, or candidate that one supports. Nearly all voting systems do not make vote trading a formal process, so vote trading is very often informal and thus not binding. One form of vote trading that is formal is one that involves the trading of proxy voting rights - party A gets Party B's voting right formally, eg as a filled in proxy form with signature, perhaps authenticated by secretariats, and in this case party A may use B's vote on issue 1, and B uses A's vote on issue 2... votes traded.
-- Wikipedia