first down

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The first in the series of four downs in which an offensive team must advance ten yards to retain possession of the ball.
• noun A gain of ten or more yards entitling the offensive team to a new series of downs.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun American football The first play in a series that has a maximum of four downs.
• noun American football The act of getting a first down by advancing the ball more than ten yards ahead of the field position where the team took possession.