What do we call the "middle" variables in a complex research model? Basic theory building goes like this: you create a theory which has an independent variable and a dependent variable. I have also been taught to use "explanatory variable" as synonym for "independent variable" and "goal variable" and "response variable" as synonyms for "dependent variable". Then you do empirical research and measure the connection strength, or find out that there is no connection after all :( 

As this is rarely complex enough for theories in disciplines where humans are involved, there is also the option to create a multivariate model, where many independent variables will explain a dependent (or goal) variable. There, the terminology is still unambiguous. 

But many models from recent publications are more complex than this. Consider this path model: 



Continuance intention seems to be the goal variable, because it doesn't lead to any other variables. Interaction and User value are certainly independent variables in this model, because they don't depend on others. But what are Satisfaction and Flow experience? They depend on lots of other variables, but they are not our goal variable, because this model is about Continuance intention, not about Satisfaction or Flow experience. 

So, what is the proper terminology for such variables stuck in the middle of a multi-level model? 

Citation for the picture: Examining users′ intention to continue using social network games: A flow experience perspective
Chiao-Chen Chang (2013)
Telematics and Informatics  30 (4) p. 311-321
I've always found it difficult to pack up real-world data into the neat boxes of methodology textbooks, but if you really need a label for these, I'd go with mediator variable.

From a famous article on these variables:


  Specifically, we differentiate between two often-confused functions of
  third  variables: (a) the moderator function of third variables, which
  partitions a focal independent variable into subgroups that establish
  its domains of maximal effectiveness in regard to a given dependent
  variable, and (b) the mediator function of a third  variable, which
  represents the generative mechanism through which the focal
  independent variable is able to influence the dependent variable of
  interest.


Reference:

Baron, R. M., &amp; Kenny, D. A. (1986). The moderator–mediator variable distinction in social psychological research: Conceptual, strategic, and statistical considerations. Journal of personality and social psychology, 51(6), 1173.