An intransitive verb is a verb that, rather than being followed by the object of a sentence, is followed by, for example, a preposition. Reply is an intransitive verb. You can’t say, I replied him; you have to use the preposition to. I replied to him.
An intransitive verb is a verb that has only one argument, that is, a verb with valency equal to one. In more familiar terms, an intransitive verb has a subject but does not have an object. For example, in English, the verbs sleep, die, and swim, are intransitive.