In the Greek translation of the Old Testament, the ineffable Hebrew name YHWH, by which God revealed himself to Moses, is rendered as Kyrios, "Lord". From then on, "Lord" becomes the more usual name by which to indicate the divinity of Israel's God. The New Testament uses this full sense of the title "Lord" both for the Father and - what is new - for Jesus, who is thereby recognized as God Himself.
Source: Catechism of the Catholic Church (Vatican.va)