signed number
A signed number is a number that wears a + or − badge telling you not just how much, but which way. Money owed versus money saved, steps up versus steps down, gain versus loss — the sign captures the direction, and the digits capture the size.
By convention a number written with no sign is taken to be positive, so 7 means +7. A negative sign in front, as in −7, reverses the direction. Zero is the neutral point that is neither positive nor negative; it is the only number whose sign does not matter. Working with signed numbers is what separates the full integers and reals from the bare counting numbers.
The sign controls the arithmetic. Two same-signed numbers add their sizes and keep the shared sign; opposite signs subtract sizes and take the sign of the larger; a product or quotient is negative when exactly one factor is negative and positive otherwise. Keeping careful track of signs is one of the most common places beginners slip, so it pays to be deliberate.
A bank balance of +20 with a charge of −30 gives 20 + (−30) = −10, an overdraft of 10.
The sign tells direction; the digits tell magnitude.