This article presents the outlooks of various Romanian linguists and of those from European and Russian area, on the opposition variant / invariant and on their importance for the language’s theory. Trying to give a schematic definition, we mention that variant represents the various outputs of one and the same entity and invariant is an abstract term, designating one and the same identity besides its manifestations and modifications. These two concepts generate the notion of language’s variability, as on their basis we can observe if a language varies (and it certainly varies and evolves), how it varies, in what manner and what kind of results brings the variation.