Definition of Air
In poker, the term "air" refers to when a player has a worthless hand.
The most common usage of the term occurs when a player bluffs another player out of a pot holding a worthless hand.

You decide to reach for some chips to put in a re-raise. After sliding out a large stack of chips, you notice your opponent looking pained. After a few moments of thought, he folds his A-J face-up. You proudly show your 2-3 offsuit and take down the pot.
In this case, you bluffed your opponent with "nothing but air".
It goes without saying that you should have a great read on your opponent if you plan on bluffing them with nothing but air.
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