A split infinitive is a grammatical construction in the English language where a word or phrase, usually an adverb or adverbial phrase, occurs between the marker to and the bare infinitive (uninflected) form of the verb. The construction is particularly notable because of some controversy as to whether it is "grammatically correct".
One famous example is from the television series Star Trek: "to boldly go where no man has gone before." Here, the presence of the adverb boldly between the parts of the infinitive, to and go , creates a split infinitive. The construction can often be avoided by placing the intervening words after the verb or before the to marker: "to go boldly where no man has gone before" or "boldly to go where no man has gone before." However, these two rephrasings do not have identical meanings - the former attaches the boldness to the manner of going, while the latter attaches the boldness to the complete act of going "where no man has gone before."
Descriptively speaking, split infinitives are common in most varieties of English. However, their status as part of the standard language is controversial. In the 19th century, some grammatical authorities sought to introduce a prescriptive rule that split infinitives should not be used in English. Most experts on language from the last 100 years, however, agree that this rule was misguided, and indeed that the split infinitive construction can sometimes help to convey one's intended meaning more accurately (as in the case of "to boldly go").