x1=s1=m1 is the material environment/surrounding material (i.e. gaseous/liquid atmosphere, vacuum, etc.) enclosing x2=s2 in direction(s)/dimensions(s)/plane x3=s3=m3, of type/composition including x4=m2.
A vacuum is arguably not 'material', but the word is much more convenient if that is included.
Also, vacuum is never completely empty. See also marji, sruri, gapci, litki, kunti, sakci, kensa.