Angelology comes from the Greek word, "angelos," which means angel and the Greek word, "logos," which means word or study. Therefore, angelology is the study of angels. The English word occurs more than 290 times in the Bible.
The Old Testament was written in Hebrew and Hebrew word for Angel is "malak." An Angel is a messenger of God. Biblically speaking, angels (sent ones who are not humans) are created beings, who are very powerful.
For more information, see the article, Angels.