In geometry, the parallel symbol is used to denote two lines that are parallel to eachother. Typically the parallel symbol is used in an expression like this:
$AB∥CD$
In plain langauge this means that the line defined by the two points $A$ and $B$ and the other line defined by the two points $C$ and $D$ are parallel to each other.
Symbol  Format  Data 

∥  Unicode 

$∥$  TeX 

SVG 