A lucky loser is a player who has lost in the qualifying draw, but still makes it into the main draw because of a withdrawal.
How common is it to be a lucky loser in a Grand Slam tournament? To put it into perspective, no player has ever been one at three Grand Slams in a row or three in the same year. Peter Polansky has broken that record, becoming the first person to have the letters (LL) by his name for all four Grand Slams in the same year.