depth: how far something reaches down, and so how far anything goes past the point where it can be measured
At its root this word is a measurement. It is the depth of soil lying over rock, too thin to keep a seedling alive once the sun comes up, and it is the deep water a fisherman rows out to when the shallows have nothing left to give. The related adjective βαθύς describes a thing as deep; this word names the depth itself.
From there it becomes the natural way to speak about anything a person cannot get to the bottom of. Paul sets it beside height, length, and breadth when he wants to name the whole reach of space, or the whole reach of Christ's love. He cries out at the "depth" of God's riches and wisdom, meaning that no one will ever finish measuring them. And he speaks of the "depths" of God in the plural: the things that lie far below the surface, which no one could dig out and which God's own Spirit alone knows.
One last use runs down a scale rather than down into space. A church in Macedonia gave generously out of poverty that went all the way to the bottom. It is still the same picture. What is deep is what you cannot see the end of, whether that is water under a boat, wealth in God's hands, or need in a poor congregation's life.