Suppose that 
 and 
 are two algebras and 
 is a unital 
-bimodule. Then
with the usual 
 matrix-like addition and matrix-like multiplication is an algebra.
An algebra 
 is called a triangular algebra if there exist algebras 
 and 
 and an 
-bimodule 
 such that 
 is (algebraically) isomorphic to
under matrix-like addition and matrix-like multiplication.
For example, the algebra  of 
 upper triangular
 matrices over the complex field 
 may be viewed as a triangular algebra when 
.