This math term for a restricting factor comes from Greek words for "beside" & "measure"
The natural world sets certain parameters,... [DOC] [TLE] Parametric - definition of parametric by The Free Dictionary [PAR] Usage Problem A factor that restricts what is possible or what results: "all the ... 
directrix of a conic : Greek para-, beside; see para-1 + Greek metron, measure; 
see -meter.