) A ''σ''-algebra
which is obtained as a "completion" of a given ''σ''-algebra, which includes all subsets of the given measure space
which simultaneously contain a member of the given ''σ''-algebra and are contained by a member of the given ''σ''-algebra, as long as the contained and containing measurable sets
have the same measure
, in which case the subset in question is assigned a measure equal to the common measure of its contained and containing measurable sets (so the measure is also being completed, in parallel with the ''σ''-algebra).