"Progressive" is used in place of liberal to best describe philosophical ideals that are opposite and contrasted to those held by conservatives. Political ideas that advocate rapid social change are likely to be progressive, while conservative ideas tend to reflect an adherence to established norms and support for (or furtherance of) status quo interests. Continuing logically, by this spectrum, a philosophy that advocated reversing course to previous standards would be regressive, though this term is rarely used. Instead, the term reactionary is more frequently used to describe those who wish to adhere to established convention.
This is particularly useful when dealing with philosophical positions, since the liberal tradition has very particular and fixed Enlightenment connotations that may not necessarily have any useful meaning in the Left political scene.
The term has its origin in American politics in the early part of the twentieth century. During this period, known as the Progressive Era, many reforms were enacted. Some third-party presidential candidates ran for office during this time under the Progressive Party label, notably Robert M. La Follette, Sr. The Progressive Party of Canada also briefly rose to prominence in the 1920s.
This term is also used in Canada, since many liberals are not Liberals; that is, they do not support the centrist Liberal Party of Canada and should not be confused with the Progressive Conservatives.