Henotheism is the practice of worshipping one God without denying that others exist or accepting that other gods exist but not worshipping them. It means to worship one God out of many. Therefore, henotheism is a subcategory of polytheism which is the teaching that many gods exist.
The word comes from the Greek 'heis' which means one and 'theos' which means God. But, this is not to be confused with monotheism which is the belief that there is only one God in all existence.
By with comparison, monolatry is a position that recognizes the existence of many gods but only serves and worships one of them, usually at a time.