In order to speak about quantity and magnitude, we have to introduce the words scalar and vector; these words and their meanings are very useful in mathematics and physics.

A vector is a quantity that has a direction and a length or magnitude, so it is a quantity specified as a list of components. If we speak about the wind speed, saying 20 mph from the East, this speed is a vector because it is specified by a magnitude of 20, and a direction from the East.

On the other hand, a scalar is a quantity that is completely described by a single number, and this number may be called magnitude. For instance, the 80 Fahrenheit degrees temperature in a room is a scalar, because it only needs to be described by a single number (80).

According to this information, the answer to your question is that a quantity completely described by its magnitude is called a scalar.