A state diagram is a type of diagram

Diagram

A diagram is a two-dimensional geometric symbolic representation of information according to some visualization technique. Sometimes, the technique uses a three-dimensional visualization which is then projected onto the two-dimensional surface...

 used in computer science

Computer science

Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

 and related fields to describe the behavior of systems. State diagrams require that the system described is composed of a finite number of state

State (computer science)

In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....

s; sometimes, this is indeed the case, while at other times this is a reasonable abstraction

Abstraction (computer science)

In computer science, abstraction is the process by which data and programs are defined with a representation similar to its pictorial meaning as rooted in the more complex realm of human life and language with their higher need of summarization and categorization , while hiding away the...

. Many forms of state diagrams exist, which differ slightly and have different semantics.
State diagrams are used to give an abstract description of the behavior

Behavior

Behavior or behaviour refers to the actions and mannerisms made by organisms, systems, or artificial entities in conjunction with its environment, which includes the other systems or organisms around as well as the physical environment...

 of a system

System

System is a set of interacting or interdependent components forming an integrated whole....

.