an expression in which there is a word or phrase, especially an adverb or adverbial phrase, between to and its accompanying verb form in an infinitive, as in to readily understand.
Origin of split infinitive
First recorded in 1895–1900
The “rule” against placing a word, especially an adverb, between to and the verb in an English infinitive ( To really learn a language, you have to stay in a place where it is spoken ) is based on an analogy with Latin, in which infinitives are only one word and hence cannot be “split.” The modeling of English style on Latin has in the past often been considered the epitome of good writing; the injunction against splitting the English infinitive is an example of the misguided application of this notion. Criticism of the split infinitive was especially strong in 19th-century usage guides. Nothing in the history of the infinitive in English, however, supports the so-called rule, and in many sentences, as in the example above, the only natural place for the modifying adverb is between to and the verb ( To really learn … ). Many modern speakers and writers depend on their ear for a natural sentence rather than on an arbitrary rule. Writers who ordinarily prefer not to split an infinitive will occasionally do so, to avoid awkward or stilted language.
British Dictionary definitions for split infinitive
(in English grammar) an infinitive used with another word between to (the infinitive marker) and the verb itself, as in I want to really finish it this time
The traditional rule against placing an adverb between to and its verb is gradually disappearing. Although it is true that a split infinitive may result in a clumsy sentence (he decided to firmly and definitively deal with the problem), this is not enough to justify the absolute condemnation that this practice has attracted. Indeed, very often the most natural position of the adverb is between to and the verb (he decided to really try next time) and to change it would result in an artificial and awkward construction (he decided really to try next time). The current view is therefore that the split infinitive is not a grammatical error. Nevertheless, many writers prefer to avoid splitting infinitives in formal written English, since readers with a more traditional point of view are likely to interpret this type of construction as incorrect
An infinitive is the “to” form of a verb, as in “to play.” A split infinitive is a phrase in which to is separated from the verb. The sentence “I decided to quickly and directly go home” contains a split infinitive. Some people consider it poor style, or even incorrect style, to split an infinitive.