If there are an odd number of competitors at any stage of a single-elimination tournament, one player is excused from play and continues on as if he had defeated his (nonexistent) opponent. This is called "getting a by". Or "getting a bye". Who knows, maybe it's even "getting a buy", although I doubt it.
My dictionary lists by as meaning "pass" in card-games, so that suggests that spelling. On the other hand, the image of the referee looking from the single player, to the empty and undefended far side of the field, and then back to the player, shrugging, and saying "Bye", is too strong to easily discard.
Like other parts of the anatomy, everyone's got an opinion. I'm looking for a cite.