Discreteness is the fact or quality of being clearly separate. If a system has discreteness, its pieces do not run into each other; you can count or name each one. Maths and science like this noun: the discreteness of steps, of atoms, of the tiny squares in a digital picture. Ordinary talk almost never needs it. People just say that things are separate. Though actually, do not mix it with discretion, which is the room to decide, or the care not to say too much. Same distant family tree; different words now.