Dictionary.com
definitions

first down

noun Football.
  1. the first of four consecutive plays during which an offensive team must advance the ball at least ten yards to retain possession of it.
  2. a gain of ten or more yards by an offensive team that entitles it to continued possession for a new series of downs.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for first down

Contemporary Examples of first down