What does " nth price auction" mean?

Context:

In experimental nth price auction, found positive
premium for non-GE food
In experimental nth price auction, found 15-percent
discount for GE food, but difference was only statisti-
cally signiicant for one of three foods
Surveyed Alabama consumers willing to pay a $0.39
or 19-21 percent premium for non-GE tomatoes
Found consumers willing to pay 14 percent less for
GE foods
In experimental auctions, consumers willing to pay 14
percent more for non-GE food
Customers willing to pay 5 percent more for non-GE
food