This page is currently being updated from the earlier version of my website. Sorry that it is not yet fully available.
Wikipedia: Intervening variable. Excerpt: An intervening variable is a hypothetical internal state that is used to explain relationships between observed variables, such as independent and dependent variables, in empirical research. An intervening variable facilitates a better understanding of the relationship between the independent and dependent variables when the variables appear to not have a definite connection. They are studied by means of operational definitions and have no existence apart. URL: en.wikipedia.org/wiki/Intervening_variable
You can find an earlier version of this page on my original website.