In the expression a ÷ b = c, a is called the dividend or numerator, b the divisor or denominator and the result c is called the quotient.
What's most common (in the context of programming)?
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Operators, operands, parameters, and arguments are usually the terms employed in programming. If any of the OP's listed terms are at all utilised, it will very likely be numerator and denominator (in their mathematical sense).