Literally (Abstract):

We define a causal mechanism as a process in which a causal variable of interest, i.e., a treatment variable, influences an outcome. The identification of a causal mechanism requires the specification of an intermediate variable or a mediator that lies on the causal pathway between the treatment and outcome variables. Although qualitative studies often employ the method of process tracing, quantitative investigation of causal mechanisms is based on the estimation of causal mediation effects. Indeed, the traditional approach to causal mediation analysis has been to use structural equation models (e.g., MacKinnon 2008; Shadish, Cook, and Campbell 2001), a practice which goes back decades (Haavelmo 1943).