A scalar is a variable that only has magnitude, e.g. a speed of 40 km/h. Compare it with vector, a quantity comprising both magnitude and direction, e.g. a velocity of 40km/h north.
Scalar (mathematics), a quantity which is independent of viewpoint, a non-tensor
Scalar (physics), the same as the notion of scalar in differential geometry
Scalar (computing), an atomic quantity that can hold only one value at a time