Constants and Variables I'm curious about a rather simple terminology question: is there an umbrella term for constants and variables? Of course, a constant does not change while a variable can, but is there a term referring simply to using a symbol to represent a value? The closest I can think of is value, but value is normally used to mean the number being represented by a constant of variable.
 A: Since Algebra uses letters to represent numbers that can vary, those letters are called variables. Some letters can be designated as constants, since their values remain constant, once assigned. Often letters are used as constants simply for the sake of convenience over using a large or complex number.
So I would simply call using a symbol to represent a value a variable in its simplest terms as the variable a for example can be used throughout an equation but can be changed in one place to a different number and so this makes it easier than simply having to write the same number in numerous places as this can cause errors. 
So in short in its simplest terms  a symbol used to represent a number is called a variable.
A: Unfortunately, I think the most common word that encompasses what you mean by both constants and variables is "variable". While some high school algebra references distinguish variables from constants (e.g. this page), it's common in other contexts for "variable" to be use for any letter/symbol that stands for a number (and occasionally even that requirement is dropped).
In a context like Calculus, variables (in this broader sense) might stand for "constants", or they might be "independent" or "dependent" variables. The text on Wikipedia at the time of this writing is fairly inconsistent and may not perfectly represent consensus in math, but this Wikipedia section regardless serves as evidence that the word "variable" can be used for your "constants". 
Note that the $\pi$ in "$\pi\approx 3.14$" is unlikely to be considered a variable, even though $\pi$ is a mathematical constant.
