After all of the votes are counted, parliament is considered "hung" if no one party ends up with enough seats to form a majority in Britain’s 650-member House of Commons.
Britain and the rest of the world won't have to wait much longer. UK election results should start to trickle out shortly after 10 pm in the UK (5 pm EST), when the polls close. While surprises are possible, opinion polls ahead of the election showed David Cameron's Conservative Party in the lead, but about 20 seats shy of a majority.
The last time the UK had a hung parliament was in 1974, when the two largest parties (the Conservatives and the rival Labour Party) lost considerable shares of the nationwide vote, thanks largely to a surge by a third force, the Liberals. The Liberals later merged with another party to become the Liberal Democrats, who are currently running third. If parliament is hung this time, the Liberal Democrats may enter into a coalition arrangement with one of the "big two."
In Britain's parliamentary system, that would leave the new government in a weak position, since a withdrawal of support by a smaller coalition partner could lead to a vote of no confidence and fresh elections. With Britain destined for a round of budget cutting and financial pain, whoever is the next premier will face no easy task fending off public anger as the head of the government will also be no easy task.
"The consequences could be dramatic," wrote the New America Foundation's Oliver Lough in an opinion piece for the Monitor earlier this year. "In a system accustomed to stable, majority government, hung parliaments generally end up both chaotic and short-lived. With parties often unable or unwilling to work toward consensus, political paralysis is a major risk. In an extreme scenario, Parliament could end up being dissolved unilaterally by the Queen."
If the Conservatives can't win over the LibDems, they may turn to the Northern Ireland’s Democratic Unionists, who have indicated they would support Cameron in exchange for promises that Northern Ireland will be spared government spending cuts that are inevitable for the rest of the country.
Failing that, the Conservatives might form what's called a "minority government" -- essentially ruling without a clear majority, though such arrangements are generally unstable and would probably lead to another election some time in the fall.
If parliament is hung and none of those options are taken, then another election could be quickly called.