Scalar

Scalar are simple number we use to do mathematical operations (e.g. addition, multiplication, …).

If we measure and do mathematical operation on length, mass, temperature or other units, it's done with scalars.

It's relevant to use the term "scalar" if we do operations on multi-dimensional tensors, like vector or matrix, so multiplications can be done even if it's not the same dimension.

For example, a scalar $k$ (1 dimension) can multiply a vector $v$ (2 dimensions).

$$k\times v=2\times [1,2,3]=[2,4,6]$$