There is not a specific word that is used to bold a number in a math equation. Generally, a number or letter is not bold in a math equation, if a letter is of significance, it is normally capitalized.
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It is each TERM of the equation or expression
math is used to show people the different ways math can be used such as fractions or decimals you name it
any number at all
They are used in place of an unknown number or value.
Possibly 0; as 0 is imperative as a placeholder for all numbers. However, it is extremely difficult to predict what number is the "most used" in (generalised?) "math".