Refers to the person or entity that plans and runs an event, conference, or competition. Can imply both organizational and financial backing.
The organizer of the tournament is a local company.
The event was canceled due to circumstances on the organizer's side.
プロモーター often implies a professional organizer of commercial events like concerts or boxing matches, while 主催者 is the general term for whoever hosts or organizes any event.
主催者 is the person or group who hosts and runs an event, while 発起人 is specifically the one who first proposed and pushed the idea forward.
主催者 is the host or sponsor who officially puts on an event, while 幹事 is the person who handles the practical planning and coordination, often on someone else's behalf.
Compound of 主催 (shusai, 'sponsorship, organization') + 者 (sha, 'person').