##### Wiktionary

**subalgebra**

n. A closed subset of an algebra.

##### Wikipedia

**Subalgebra**

In mathematics, a **subalgebra** is a subset of an algebra, closed under all its operations, and carrying the induced operations.

" Algebra", when referring to a structure, often means a vector space or module equipped with an additional bilinear operation. Algebras in universal algebra are far more general: they are a common generalisation of *all* algebraic structures. Subalgebra can be a subset of both cases.