lord: the one who rightfully possesses, protects, and commands those under his care
At its center this word names the rightful authority of whoever stands at the head of a sphere of life: the master over his servants and household, the owner over his property, the husband or father within his family, the ruler over his land. Underneath every use is a single relationship seen from below: there is one who commands and provides, and there is one who owes him submission and honor.
The word's whole range is one stable idea stretched across spheres of different size. The slave-master, the lord of a household, the lord of a nation, and the God of Israel are all called by this one word in the same sense: each is the one to whom submission is due within his realm. What grows across the range is not the meaning but the size of the realm, until in the prophets the realm becomes the whole earth and the word names the only Lord there ultimately is.
Because that authority is always felt from below, the word also became the ordinary courteous way to address anyone above you. A servant says it to his master, but so does a man to a stranger he wants to honor, a wife to her husband, a subject to a king. In those settings it works much like the English "sir": it grants deference without necessarily claiming the speaker is owned.
When the word is used of God it carries its full weight. Scripture calls the God of Israel "the Lord" with this word, especially in the sweeping titles "the Lord of all the earth" and "the Lord, the LORD of hosts." The everyday picture of a master who rightly commands his household is here pressed to its furthest reach: the one to whom the whole world answers.