In the field of computation theory, union is one of the regular operations. If `A` and `B` are languages (not necessarily regular languages) then the union of `A` and `B` (usually written `A` ∪ `B`) is defined as follows.

{`x`
| `x` ∈ `A` or `x` ∈ `B`}

In plain English: `A` ∪ `B` is the set of all strings which are either members of `A` or `B` or both.

For example, if `A` = {a,b,c}, and `B` = {c,d,e}

`A` ∪ `B` = {a,b,c,d,e}

Some interesting things to note about the union operation:

- The union of a set with any subset is equal to the original set (iff
`B` ⊆ `A`, then `A` ∪ `B` = `A`).
- Regular languages are closed under the union operation. This means that if
`A` and `B` are regular languages, then so is `A` ∪ `B`.