A counterrevolutionary is anyone who opposes a revolution, particularly those who act after a revolution to try to overturn or reverse it, in full or in part.
Monarchists and supporters of the ancien regime following the French Revolution were counterrevolutionaries, and so were the monarchies that put down the various Revolutions of 1848.
In France before World War I, people who "opposed democratic ideas, parliamentary government, trade-unions, or socialism" were often considered counterrevolutionary by their opponents. The White Army and its supporters who tried to defeat the Bolsheviks after the October Revolution, as well as the German Freikorps who crushed the German revolution of 1919, were also counterrevolutionaries.
More recently, the 1961 Bay of Pigs invasion into Cuba was conducted by counterrevolutionaries who hoped to overthrow the revolutionary government of Fidel Castro. In the 1980s, the United States sponsored Contra rebels fighting to overthrow the revolutionary Sandinista government in Nicaragua. In fact, the Contras received their name precisely because they were counterrevolutinaries.
Some counterrevolutionaries are former revolutionaries who supported the initial overthrow of the old regime but came to differ with those who ultimately came to power after the revolution. Thus, some of the Contras had fought with the Sandinistas to overthrow Anastasio Somoza and some of those who oppose Castro also opposed Batista.
The word is often used interchangeably with reactionary; however, some reactionaries (like the Nazis and Italian fascists) used the term counterrevolutionary to describe their opponents - even if those opponents were advocates of a Marxist revolution. Similarly, the clerics who took power following the Iranian Revolution would describe all those who opposed them as counterrevolutionary, even though some were Communists. The term, therefore, should be understood in a relative sense rather than as an absolute concept.
(1) Liberalism and the Challenge of Fascism, Social Forces in England and France (1815-1870), Prof. J. Salwyn Schapiro, McGraw-Hill Book Co., Inc., NY, l949. pg 364.