The use of the term of Protestantism is later than the reforms of I6th Century and we have to wonder about the scope and the meaning of this term beyond the diversity of the Churches and movements to which it corresponds to. Moreover the Protestants often dont refer to themselves first and foremost as Protestants but as Reformed, Lutheran, Baptist, Evangelical, …In studying the uses of the words protestant and protestantism, the author shows that through the usage of these terms, the Protestants try, ad intra and ad extra, to express their unity beyond their denominational, theological and ecclesiological diversity and to mutualize their actions and representations. In refering to the events of the I6th Century, a common identity is little by little created, a common identity constantly questionned about their essential components and about their délimitations. The preparing of the commémoration of the 500 years of the Reformation reactivates this questionning about what is in the heart of the affirmation of a Christianism different from the Roman Catholicism and from the Orthodoxy.