In government
Government refers to the legislators, administrators, and arbitrators in the administrative bureaucracy who control a state at a given time, and to the system of government by which they are organized...

, unicameralism (Latin , one + , chamber) is the practice of having one legislative or parliamentary chamber. Thus, a unicameral parliament or unicameral legislature is a legislature
A legislature is a kind of deliberative assembly with the power to pass, amend, and repeal laws. The law created by a legislature is called legislation or statutory law. In addition to enacting laws, legislatures usually have exclusive authority to raise or lower taxes and adopt the budget and...

 which consists of one chamber or house. Some countries with unicameral legislatures are often small and homogeneous unitary states and consider an upper house or second chamber unnecessary.
Unicameral legislatures exist when there is no widely perceived need for multicameralism
In contrast to unicameralism, multicameralism or 'polycameralism' is the condition in which a legislature is divided into several deliberative assemblies, which are commonly called "chambers" or "houses". This can include bicameralism with two chambers, tricameralism with three, tetracameralism...