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
x1 (collection, body, set, mass, tuple, n-some, etc.) is x2 (li; default: 1) indivisible/atomic/elementary/basic discrete entities (or particles) of type x3 in composition/content, by standard x4; the count of instances of x3 in x1 is x2 by standard x4.
x1 is the standard variation of/in data set/distribution x2; x1 is the quantified (in)accuracy of the results x2 given by some observation or measurement (procedure), calculated from/by standard x3