When talking about vectors, sometimes it can get confusing
because one also talks about real numbers like 
or
e at the same time. When mentioning good old regular numbers
at the same time as vectors, it's useful to give these
regular numbers a name to distinguish them from vectors.
We call them scalars. Well actually there's a bit more
to it than that. I'm being awfully cavalier. To explain
this subtle point, take as an example the length of
the vector 
above, so called 
. Note that
when we rotated the coordinate system,
didn't change. So that's a scalar. Now if instead you just
look at the x component of a vector, then we saw that
this did change. So you wouldn't want to call 
a
scalar, though it is just a regular number. You might
want to keep this subtlety in mind, though it probably
won't come up very much here.