In rugby union, the player who traditionally wears number one is the loosehead prop, commonly shortened to prop. The number is part of the standard starting-lineup numbering system used in rugby union.
Props occupy the front row of the scrum alongside the hooker, who normally wears number two, and the tighthead prop, who normally wears number three. The loosehead prop binds on the left side of the scrum from the team’s perspective, while the tighthead prop lines up on the right. Their main jobs include providing stability, generating force, and supporting the hooker during the scrum.
A common mix-up is assuming that jersey numbers identify rigid positions in every form of rugby. Rugby league uses different positional conventions, and shirt numbers can vary in youth, amateur, or promotional matches. In a standard rugby union starting side, however, number one is the loosehead prop. Therefore, “prop” is the fair canonical answer to this question, even though “loosehead prop” is more precise.