to have: to hold something as one's own, on oneself, or within one's situation, whether a possession, a relationship, an inner reality, or a state one is in.
This is one of the most common words in the New Testament, and almost all of its uses come back to a single idea: to have. What is had can be a thing you own, a person you are bound to, something carried on your body, or something as inward as life, faith, or sin. The English word "have" stretches the same way, which is why this verb so often disappears into a plain translation.
The senses below are not different words; they are the natural reach of one idea. You can have a field, have a wife, have a sword on your hip, have authority, have a fever, or have nothing to pay with, and in Greek it is the same verb each time. What changes is what is being had and how. Two patterns drift furthest from "own": when the verb is followed by a description it can mean to hold or count someone as something (to have John as a prophet), and when it is followed by a word for how a person is doing it slides into simply being in that state (the sick are literally those who "have badly").
In John's Gospel and letters the word carries real weight. To "have" eternal life, to "have" the Son, to "have" fellowship is not loose talk: it states present, settled possession of the thing that matters most. The flat everyday verb becomes a way of saying that the gift is already held, not merely hoped for.