A revolutionary is a person who either actively participates in, or advocates revolution

Revolution

A revolution is a fundamental change in power or organizational structures that takes place in a relatively short period of time.Aristotle described two types of political revolution:...

. Also, when used as an adjective, the term revolutionary refers to something that has a major, sudden impact on society or on some aspect of human endeavor.
The term —both as a noun and adjective— is usually applied to the field of politics

Politics

Politics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...

, and is occasionally used in the context of science

Science

Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

, invention

Invention

An invention is a novel composition, device, or process. An invention may be derived from a pre-existing model or idea, or it could be independently conceived, in which case it may be a radical breakthrough. In addition, there is cultural invention, which is an innovative set of useful social...

 or art

Art

Art is the product or process of deliberately arranging items in a way that influences and affects one or more of the senses, emotions, and intellect....

. In politics, a revolutionary is someone who supports abrupt, rapid, and drastic change, while a reformist

Reformism

Reformism is the belief that gradual democratic changes in a society can ultimately change a society's fundamental economic relations and political structures...

 is someone who supports more gradual and incremental change.