one's exclusive domain; unchallenged field; monopoly
Refers to a situation or area where one person or group dominates completely, with no rivals. Often used in competitive contexts like sports, business, or entertainment. The word originated from a confusion between 擅 and 壇, but 独壇場 is now the standard form.
See also: 独擅場
He has built an unchallenged position in that field.
In this market, that company has a monopoly.