If I needed to describe the distance between
two cities, I could provide an answer consisting of a single
number in miles, kilometers, or some other unit of linear
measurement. However, if I were to describe how to travel
from one city to another, I would have to provide more
information than just the distance between those two cities;
I would also have to provide information about the
direction to travel, as well.
The kind of information that expresses a
single dimension, such as linear distance, is called a
scalar quantity in mathematics. Scalar numbers are the
kind of numbers you've used in most all of your mathematical
applications so far. The voltage produced by a battery, for
example, is a scalar quantity. So is the resistance of a
piece of wire (ohms), or the current through it (amps).