"Pure nitrogen" refers to nitrogen that is not mixed or combined with any other substance. It is nitrogen in its purest form, with no impurities. Full definition
As the question says, does filling pure nitrogen has any significant advantages, given the fact that normal air is ~ 80 % nitrogen? (mechanics.stackexchange.com)
Nitro Merlin Milk Stout cans are directly pressurized with pure nitrogen gas, without use of the «widgets» that are often employed in canned nitro beers. (brewbound.com)
When trying to maximize fuel economy, some consider filling their tires with pure nitrogen instead of atmospheric air (scientificamerican.com)