Host
Hosting, Sponsor
noun, verbal noun
The master sponsor for an event is the host who's hosting (and possibly sponsoring) it.
主催 is a formal term often used in business, official events, or public gatherings. While it can imply sponsorship, it mainly refers to the act of leading and organizing an event or activity, as well as the person or group hosting it.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
芸能人主催のゴルフ大会ってどんなのなんだろう？
I wonder what a golf tournament hosted by a celebrity is like.
このイベントは、地元の商工会が主催しています。
This event is hosted by the local chamber of commerce.
実はこのコンサートの主催は、とある有名な音楽プロデューサーなんです。
Actually, the host of this concert is a well-known music producer.
競馬場の芝についてのシンポジウムが市当局の主催で開かれた。
The racetrack turf symposium was able to happen because of city government sponsorship.