In football — the game most of the world calls football, and Americans call soccer — a fullback is a defender. The two fullbacks usually play on the left and right of the back line. Their first job is to stop the other team getting through, though modern fullbacks also run forward and help attack. Rugby has a fullback too: the last player standing at the back, who catches high kicks and makes tackles. Though actually, in American football a fullback is an attacking player who blocks and sometimes runs with the ball — almost the opposite job. If you meet the word in a Premier League report, think defender; if you meet it in the NFL, think a short, strong attacker just behind the quarterback. It is written as one word. Neutral sports English.