an outermost edge: the farthest point something reaches, above all the farthest reaches of the earth
At its most concrete this word names the place where something stops, the edge past which it does not go. That is why it can also measure a body. When Ezekiel is led out into the temple river, the water first comes only to his ankles, the place where his legs end, before it rises to the knees, the waist, and finally a current too deep to cross.
Its most common use by far is the fixed phrase "the ends of the earth": the world stretched out to its farthest horizon in every direction. Kings in the ancient Near East liked to claim they ruled to the ends of the earth, and Israel's poets take that language back for God and for God's king. The LORD judges the ends of the earth (1 Samuel 2:10), his rule runs that far (Psalm 72:8), and every last corner of the world sees his rescue (Psalm 98:3, Isaiah 52:10). In Isaiah the phrase even turns into an invitation rather than a boast: "turn to me and be saved, all the ends of the earth" (Isaiah 45:22).
Scholars disagree over how literally to take that phrase. Some hear grand poetry, a sweeping way of saying "everywhere," pointing out that in Psalm 72:8 it stands beside concrete geography ("from sea to sea, and from the River") inside a poem full of hyperbole. Others hear a pointed answer to the Mesopotamian kings who styled themselves rulers of the four quarters of the world. Either way, this sense and the next one share a single idea: the end of a thing is the point at which it ceases to be. The old Greek translators reached for their own word for a boundary or farthest limit (πέρας) to carry it.