Kurios (2962), properly an adjective, signifying "having power" (kuros) or "authority," is used as a noun, variously translated in the NT, "`Lord,' `master,' `Master,' `owner,' `Sir,' a title of wide significance, occurring in each book of the NT save Titus and the Epistles of John.
(from Vine's Expository Dictionary of Biblical Words)
(Copyright (C) 1985, Thomas Nelson Publishers)