Parameters

In a function, we may have variables, constants, and parameters. For example, in the function

B is the function name, or dependent variable.

t is the independent variable; the number of minutes used.

15 is a constant; here it's the monthly access charge.

M is a parameter; it represents the per-minute charge.

So when we look at the graph of this function, we want to see how the monthly bill changes as the number of minutes used changes. But that may depend on M, the per-minute charge. The idea is that we may want to fix M and look at the graph, and then fix M at something different, and look at the graph to compare it with the first one. (See also Graph on the Same Axes.) M is not a constant, because we can change it. But it's not really a variable, either, since we're interested in 1 independent (here, t) and 1 dependent (here, B) variable. So we call it a parameter.