Nuisance variable
In the theory of stochastic processes in probability theory and statistics , a nuisance variable is a random variable that is fundamental to the probabilistic model , but that is of no particular interest in itself or is no longer of interest: one such usage arises for the Chapman–Kolmogorov equation . 
<triplet> random variable <subj> probability theory <obj> part of <subj> statistics <obj> facet of