Expletives are words that take the place and function of other words. They are part of a language's grammar, though not all languages have expletives. They serve to take the place of subjects or to introduce the sentence. Consider this example:

"It is important for you to remember that all pronouns should have a clearly defined antecedent."

Bishop Robert Lowth objected that since it is a pronoun, it should have an antecedent. Since it cannot function like that in Latin, Lowth said that the usage was incorrect in English.

He was wrong. Not only is English not Latin, but It is not a pronoun in the example sentence; it is an expletive. It takes the place of the verb's non-existent (or perhaps hearsay) subject and serves no other purpose.

It is worth noting that Bishop Lowth did not condemn sentences that use there as an expletive, even though in certain contexts, there is a pronoun that refers to an already-identified place.

In a sentence like "There are ten desks here" the word order is object-subject-verb-object (or expletive-verb-subject-object). The subject, desks, is after the obvious verb, are. Here is the subject. What is there doing? It is an expletive. It introduces the sentence, meaning roughly, "introducing this sentence to you".

The term expletive is also used to refer to "bad language". In the Watergate tape transcripts, the phrase expletive deleted refers to an obscene or otherwise offensive term. Expletives in this sense are adjectives, nouns or, most commonly, interjections, rather than the grammatical expletives discussed above. However, they often share the characteristics of lacking meaning and grammatical function.