Specify an interaction term. Let Y be your dependent variable and X1 and X2 two explanatory variables. Run the following regression:
Y = B0 + B1*X1 + B2*X2 + B3*X1*X2
B3 is the one you want to interpret. It says how much the effect of X1 is moderated by the levels of X2 (or, how much the effect of X2 is moderated by the levels of X1).
Yes, you can easily call that variable a moderator or moderating variable. In my equation above, X2 can be said to be the moderator of the effect of X1.
Kristian