Oracle Text

Flying (This creature can't be blocked except by creatures with flying or reach.)

Nightmare's power and toughness are each equal to the number of Swamps you control.

Card Rulings

4/1/2008 If you control 0 swamps, then the Nightmare has 0 toughness and will be put into its owner’s graveyard as a state-based action right before the next player gains priority.
10/1/2009 Nightmare’s power and toughness changes as the number of Swamps you control changes.
7/1/2013 The ability that defines Nightmare’s power and toughness works everywhere, not just on the battlefield.
7/1/2013 Nightmare’s ability counts all lands you control with the subtype Swamp, not just ones named Swamp.