Concise Bible Dictionary:

In the Old Testament there are five words so translated, but only one that occurs frequently, adon, which is often rendered “Lord,” and signifies “master” either as owner or ruler. In the New Testament there is 1. κύριος, often translated “Lord”; this is rendered “master” in reference to God in Ephesians 6:99And, ye masters, do the same things unto them, forbearing threatening: knowing that your Master also is in heaven; neither is there respect of persons with him. (Ephesians 6:9) and Colossians 4:11Masters, give unto your servants that which is just and equal; knowing that ye also have a Master in heaven. (Colossians 4:1); and is often used as between master and servants.