In practice, I don't think anyone really distinguishes between the two. Certainly the ST7 Core Reading doesn't concern itself with it, so I shouldn't worry in the exam which terminology you use.
Generally speaking, I'd say that a variable is the thing you are estimating (eg average cost per claim). A parameter would be anything you think affects that variable (eg claims inflation).
Mind you, I've heard different explanations from different actuaries. Some people would say that a parameter is simply the value you attribute to a variable when you are building a model (eg if you're including claims inflation in your model then this would be a variable, and you might set the parameter for claims inflation to be 3%).
Last edited: Aug 27, 2015
