I know this is a very elementary question, but I suddenly realized in calculus that I don't really know precisely what the definition of a variable and constant was. I know what people tend to call constants and variables in something like: ax + by = c, where you'd call x and y a variable and a,b,c constants. ...But aren't a and b subject to change just as much as x and y? And x and y just represent a SINGLE VALUE, not many values! They don't "vary". So isn't everything a constant? x is supposed to represent some number, or in other words, some CONSTANT. Also, why is it that in ∫ f(x)dx = F(x) + C, C is called the constant while x is a variable?