 Algebras are a particular type of structure or model. An \(L\)-algebra is a structure or model for the \(L\)-algebraic first-order language. Therefore the concepts of model theory for the first-order languages apply to them (see the entries on  classical logic  and  first-order model theory).  We need some of these concepts. They are also used in universal algebra, a field that to some extent can be considered the model theory of the algebraic languages. We introduce the definitions of the concepts we need.