championship-deciding contest; final match; the finals
Used in sports contexts for a match or contest that determines the champion or title holder.
The championship-deciding match will be held tonight.
決勝戦 is a general term for a final match in a tournament, while 王座決定戦 specifically implies a match to decide the champion or title holder, often in a context where the title is vacant or challenged.
Compound of 王座 (throne, championship) + 決定戦 (deciding match).