What is meant by a|a notation?

Question

Is the "divides" relation on set of positive integer reflexive?

In solution of above question I found following.

Because a|a whenever a is a positive integer,the "divides" relation is reflexive.

My question is what does a|a mean?

Answer 1

If $m$ and $n$ are integers, $m\mid n$ means that there is an integer $k$ such that $n=km$, i.e., that $n$ is a multiple of $m$, or $m$ is a divisor of $n$ (‘$m$ divides $n$’). In particular, if $a\in\Bbb Z^+$, then certainly $a=1\cdot a$, so $a\mid a$. The divides relation is therefore reflexive on $\Bbb Z^+$.