Summary: In about 2500 words the concept of opposition is at first defined and examined from a theoretical perspective, then its different functions and forms are considered also according to some classification attempts and finally a reflection on the contemporary majority-opposition relationships is proposed.
In 18th century England, the term indicated a political behaviour referring to each parliamentary party setting itself in contrast with the governement, later defined by the Oxford dictionary as "any party or body of opponents".
A wide digression on the theoretical implications of the concept is presented in order to stress how the very meaning of opposition is substantively linked to the Constitution core and it does not question the basis of a political system, all revolutionary and extralegal actions being other phenomena.
Different forms of opposition can emerge, among others, according to how the conflict potential is politicized (Kirchheimer's opposition of principle, a conflict solving model as theorized e.g. by Lijphart or Dahl, a social opposition integrating process, etc...), or according to the form of governement (presidential or parliamentary).
Then the structural classification of opposition proposed by Robert Dahl in 1966 in the case of hegemonies, polyarchies or mixed systems, is largely discussed and some further analysis are mentioned (e.g. Oberreuter's).
Finally the author reflects on the necessity to guarantee the role of the opposition in order to maintain a functioning and effective dualism between the former and the governement.