X is the symbol used to denote the independent input variables to a process. These inputs could be material, manpower and/or machine and its settings etc. A combination of these Xs determine the output of the process and is usually represented by f(X) and read as 'function of x'.
Y is the symbol used to denote the dependent output variable from a process. This output is usually a metric which is monitored for consistent performance (e.g. throughput, cycle time, quality, productivity etc.). The output is related to the inputs by the equation Y = f(X) and read as 'Y is a function of X'.
An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Prashanth Datta on 25th January 2019.
Applause for all the respondents- Prashanth Datta, Hari Shyam Gupta, Santosh Chaurasiya, Vastupal Vashisth, Mathi, Shashank Parihar, Purbi Adak.