The person who plans matches and storylines. The scriptwriter of wrestling. In WWE, this has historically meant Vince McMahon (now Triple H). In AEW, it's Tony Khan. The booker decides who wins, who loses, who gets pushed, and who jobs. Being a booker who also wrestles is a notorious conflict of interest — see Triple H's Reign of Terror, 2002-2005. Every bad storyline is the booker's fault. Every good one, the wrestlers take credit for.
"Who booked this? He's been champion for eight months and can't draw a crowd at a house show."