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.
x1 is an eigenvalue (or zero) of linear transformation/square matrix x2, associated with/'owning' all vectors in generalized eigenspace x3 (implies neither nondegeneracy nor degeneracy; default includes the zero vector) with 'eigenspace-generalization' power/exponent x4 (typically and probably by cultural default will be 1), with algebraic multiplicity (of eigenvalue) x5
x1 (ordered list) is a sampling of entries of matrix/tensor x2 in which exactly one entry is sampled from each row and/or column (etc.) between entries x3 (list; default: the largest 'square'/'hypercubic' sampling possible in the entire tensor starting with the first entry, see notes) inclusively following selection procedure/rule/function/order x4 (default: diagonally, see notes), where the tensor/matrix is expressed in basis/under conditions x5