In linguistics, blocking refers to the morphological phenomenon in which a possible form for a word cannot surface because it is "blocked" by another form whose features are the most appropriate to the surface form's environment. More basically, it may also be construed as the "non-occurrence of one form due to the simple existence of another."
Word formation employs processes such as the plural marker in English s or es (e.g. dog and dogs or wish and wishes). This plural marker is not, however, acceptable on the word child (as in *childs), because it is "blocked" by the presence of the competing form children, which in this case inherits features from an older morphological process.
One possible approach to blocking effects is that of distributed morphology, which asserts that semantic and syntactic features create slots or cells in which items can appear. Blocking happens when one cell is engaged by one form as opposed to another. Blocking has been explained along two primary dimensions: the size of the blocking object, and the existence of ungrammatical forms.