split infinitive

Concise Oxford English Dictionary © 2008 Oxford University Press:
split infinitive

▶noun a construction consisting of an infinitive with an adverb or other word inserted between to and the verb, e.g. she seems to really like it.

usage: It is still widely held that splitting infinitives is wrong, a view based on an analogy with Latin. In Latin, infinitives cannot be split as they consist of only one word (e.g. amare ‘to love’). It is therefore said that they should not be split in English either. But English is not the same as Latin, and the avoidance of a split infinitive can change the emphasis of a sentence or sound awkward (as in she seems really to like it). For this reason, the rule about avoiding split infinitives is not followed so strictly today, although it is best not to split them in formal writing.