one: the number one, and from it the idea of being a single thing, or being unified into one.
At heart this is just the number one, the word you use to count a single item among many: one sheep out of a hundred, one coin, one hour. From that plain counting sense the word reaches in several natural directions, and the New Testament uses every one of them.
It can simply pick out a single instance ("a" or "a certain" person), much the way English uses "a." It pairs with itself to sort things into groups ("one... and another"). And in a set phrase it marks the first day of the week, the day of the resurrection.
Its weightiest use is the leap from counting to oneness. To say two people become "one flesh," that many members form "one body," or that Father and Son "are one" is still the number one, but now it means undivided unity rather than a tally. This is also the word behind the Bible's bedrock claim that God is one: not one god among several, but the single God there is. Plain counting and profound oneness are the same word, and the reader has to feel which is in view.