Define $e_i \in \R ^n$ by $[e_i]_j = 1$ if $i
= j$ and $0$ otherwise.
Then $e_1, e_2, \dots , e_n \in \R ^n$ are
called the standard basis
vectors (canonical basis
vectors) for $\R ^n$.
For example, in $\R ^3$,
\[
e_1 = \bmat{1 \\ 0 \\ 0}
\]