The Lebesgue measure is the usual measure on the real line (or on any Cartesian space). It may be characterised as the Radon measure which is translation invariant and assigns measure to the unit interval (or unit cube). It generalises (up to a scalar constant) to Haar measure on any locally compact topological group.
The Lebesgue measure’s origins can be traced to the broader theory of Lebesgue integration. The original purpose of the latter, in broad terms, was to expand the class of integrable functions in order to give meaning to functions that are not Riemann integrable. In order to accomplish this, the basic properties of the concept of the length of an interval must be understood. This then leads to the need to fully define the concept of measure, particularly in relation to sets. We begin with a lemma and a corollary.
Let I be an interval, where are disjoint intervals. Then (interpreted so that can be , either because one of the summands is or because the series diverges).
If I is any interval, then
where is any countable covering of I by intervals.
Now suppose is an arbitrary set of real numbers. In order for to be measurable, we must have where is any countable covering of by intervals. We must also have where the infinum is taken over all countable coverings of by intervals.
We define the Lebesgue outer measure? as follows:
The set is Lebesgue measurable if
holds for every set . Restricting to these sets, Lebesgue outer measure becomes an honest measure.
Note that once the Lebesgue measure is known for open sets, the outer regularity property allows us to find it for all Borel sets (but also rather more sets).
For a very intuitive and readable derivation, see: