The colon symbol is used to syntactically to represent a such that statement in mathematics. The colon symbol is also used to represent the ratio between two expression. Typically, the symbol is used in an expression like the following expression which defines the set of complex numbers:
In plain language, this means that a member of exprssion such that and belong to the set of real numbers and .
Symbol  Format  Data 

:  Unicode 

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The colon symbol is used in math to denote a condition for a statement. The symbol can be read as "such that" in a math expression.
The syntax for a ratio is two numbers separated by the colon symbol.