Definition []. Crase (from greek krâsis, "mixture", "blend") refers to the contraction of two vowels.In Portuguese, only two vowels "a" can make up a crase, the first "a" being the preposition a (to, at, in, for) and the second "a" must be the first character of the next word. This phenomenon occurs in the following situations: The preposition a before the definite article a ("the", feminine ...