A marginal seat is an electorate where the winner at the most recent election beat their closest opponent by six per cent of the vote or less - after preferences.
That's the figure the Australian Electoral Commission uses to define a "marginal seat"*.
In other words, only about six per cent of voters have to change their minds for a different candidate to win.
So political parties tend to target marginal electorates because they can win them by converting the fewest voters.
University of Queensland senior lecturer in political science Dr Rae Waer says that's why parties spend so much time and money in marginal or "key" seats.
"What happens is that the major parties go into those seats and they poll intensively, they conduct focus groups in those seats and they get a real sense of what voters in those seats want. So they are much more concerned with the opinions of voters in those seats than they are with voters in more secure seats elsewhere."
Dr Waer says sometimes, that's why issues that are only important to a small proportion of Australia's population gain national prominence during an election campaign.
She believes that's the reason behind the focus on asylum seeker policy.
"We are seeing a skewing where clearly some marginal seats in New South Wales are really slanting the debate on asylum seekers. And so they target their policies to them because they want to win those seats. "
Canberra University's assistant professor in government Dr Robin Tennant-Wood says a similar thing is true of the focus on car manufacturing.
"Labor holds Corangamite (in Victoria) by just a handful of votes and with the potential closure of the car factories down there we've seen both parties trying to pour money into the auto industry. It's got nothing to do with propping up the auto industry. It's got everything to do with grabbing those last votes.
Labor holds 26 seats on a margin of 6 per cent or less while the Coalition has 29.
They each have three MPs who got into parliament by margins of less than one per cent in 2010.
But Australian National University professor of politics John Uhr says marginal seats may not be as important as they once were.
He says the minority Labor government has dramatically changed the nation's political landscape and the old rules may need to be thrown out .
"The last three years has been unlike any other last three years and honestly for anyone to tell you that they can identify the key seats is, it's old hat. It's yesterday's story. I think we're into a new world of politics."
University of Queensland's Dr Waer agrees.
The Electoral Commisson defines a seat as "safe" if it was won by 10 per cent or more of the vote, after preferences, at the previous election.
Dr Waer says this definition of a safe seat won't necessarily hold true at this election, with predictions of swings of more than 10 per cent in some electorates.
