The english word demon comes from the greek word, daemon (δαίμων). According to "Thayer's Greek-English Lexicon of the New Testament" by Joseph Henry Thayer, D.D. demon is defined as "a god, a goddess; an inferior diety, whether good or bad" and according to "The Analytical Greek Lexicon" (published by Zondervan Publishing House in Grand Rapids, Michigan. 7th Printing 1972. Catalog # 6257) demon is defined as "a god, a superior power; in N.T. a malignant demon, evil angel."
Authors of the new testament bible books and letters use the term "demon" to reference evil entities, but in actual greek use it refers to any beings which are more powerful than humans, regardless of whether the beings are good or bad, though it seems that the greek use of the word also relates that the being should or must be worshipped as a diety to fit this definition. For example, angels as depicted in the christian bible would fit under the descriptive use of the term demon because they are (according to both biblical and non-biblical sources throughout history) more powerful than humans are, and some angels were worshipped as dieties.