Lebesgue measurability. Definition.

We want to describe a class of sets such that the Lebesgue outer measure obeys nice properties. Every subset of ${\mathrm{R}}^d$ has a Lebesgue outer measure (possibly infinite). Unfortunately, it turns out that if we include all sets, we lose properties like the union of some disjoint sets has measure which is the sum of the individual sets.

With this in mind, we wish to choose a criteria which will create a restricted class of sets. The below definition is one way to choose the criteria. Note how it both acts as a criteria and as a useful property describing the limits/capabilities of Lebesgue measure for such sets.