In mathematical analysis and in probability theory, a σ-algebra ("sigma algebra") is part of the formalism for defining sets that can be measured. In calculus and analysis, for example, σ-algebras are used to define the concept of sets with area or volume. In probability theory, they are used to define events with a well-defined probability. In this way, σ-algebras help to formalize the notion of size.

In formal terms, a σ-algebra (also σ-field, where the σ comes from the German "Summe", meaning "sum") on a set ${\displaystyle X}$ is a nonempty collection ${\displaystyle \Sigma }$ of subsets of ${\displaystyle X}$ closed under complement, countable unions, and countable intersections. The ordered pair ${\displaystyle (X,\Sigma )}$ is called a measurable space.

The set ${\displaystyle X}$ is understood to be an ambient space (such as the 2D plane or the set of outcomes when rolling a six-sided die {1,2,3,4,5,6}), and the collection ${\displaystyle \Sigma }$ is a choice of subsets declared to have a well-defined size. The closure requirements for σ-algebras are designed to capture our intuitive ideas about how sizes combine: if there is a well-defined probability that an event occurs, there should be a well-defined probability that it does not occur (closure under complements); if several sets have a well-defined size, so should their combination (countable unions); if several events have a well-defined probability of occurring, so should the event where they all occur simultaneously (countable intersections).

The definition of σ-algebra resembles other mathematical structures such as a topology (which is required to be closed under all unions but only finite intersections, and which doesn't necessarily contain all complements of its sets) or a set algebra (which is closed only under finite unions and intersections).

If ${\displaystyle X=\{a,b,c,d\}}$ one possible σ-algebra on ${\displaystyle X}$ is ${\displaystyle \Sigma =\{\varnothing ,\{a,b\},\{c,d\},\{a,b,c,d\}\},}$ where ${\displaystyle \varnothing }$ is the empty set. In general, a finite algebra is always a σ-algebra.

If ${\displaystyle \{A_{1},A_{2},A_{3},\ldots \},}$ is a countable partition of ${\displaystyle X}$ then the collection of all unions of sets in the partition (including the empty set) is a σ-algebra.

A more useful example is the set of subsets of the real line formed by starting with all open intervals and adding in all countable unions, countable intersections, and relative complements and continuing this process (by transfinite iteration through all countable ordinals) until the relevant closure properties are achieved (a construction known as the Borel hierarchy).

There are at least three key motivators for σ-algebras: defining measures, manipulating limits of sets, and managing partial information characterized by sets.