Empty category
In linguistics, an empty category, which may also be referred to as a covert category, is an element in the study of syntax that does not have any phonological content and is therefore unpronounced. Empty categories exist in contrast to overt categories which are pronounced. When representing empty categories in tree structures, linguists use a null symbol (∅) to depict the idea that there is a mental category at the level being represented, even if the word(s) are being left out of overt speech. The phenomenon was named and outlined by Noam Chomsky in his 1981 LGB framework, and serves to address apparent violations of locality of selection — there are different types of empty categories that each appear to account for locality violations in different environments. Empty categories are present in most of the world's languages, although different languages allow for different categories to be empty. (Wikipedia).
