A split infinitive is a grammatical construction in which an adverb or adverbial phrase separates the "to" and "infinitive" constituents of what was traditionally called the "full infinitive", but is more commonly known in modern linguistics as the to-infinitive (e.g. to go). In the history of English language aesthetics, the split infinitive was often deprecated, despite its prevalence in colloquial speech. The opening sequence of the Star Trek television series contains a well-known example, "to boldly go where no man has gone before", wherein the adverb boldly was said to split the full infinitive, to go.
Multiple words may split a to-infinitive, such as: "The population is expected to more than double in the next ten years."
In the 19th century, some linguistic prescriptivists sought to disallow the split infinitive, and the resulting conflict had considerable cultural importance. The construction still renders disagreement, but modern English usage guides have largely dropped the objection to it. (Full article...)