I searched a lot for you. There is no formal and unique definition, however, by considering MSTAT-C term, I found that this program is written in C programming language so, the last "C" could be related to this. Also "STAT" is abbreviation of statistics , moreover, "M" could be related to a)Michigan State (where the software is produced) or b)Microcomputer. I think the possible means of MSTAT-C is :Microcomputer Statistics written in C programming language