The colon symbol is used to syntactically to represent a such that statement in mathematics. The colon symbol is also used to represent the radio between two expression. Typically, the symbol is used in an expression like the following expression which defines the set of complex numbers:
$C={a+bi:a,b∈Randi=−1 }$
In plain language, this means that a member of exprssion $a+bi$ such that $a$ and $b$ belong to the set of real numbers and $i=−1 $.
Symbol  Format  Data 

:  Unicode 

$:$  TeX 

SVG 