An angel is a supernatural being or spirit, often depicted in humanoid form with feathered wings on their backs and halos around their heads, found in various religions and mythologies. The theological study of angels is known as "angelology". In Zoroastrianism and Abrahamic religions they are often depicted as benevolent celestial beings who act as intermediaries between Heaven and Earth, or as guardian spirits or a guiding influence. The term "angel" has also been expanded to various notions of spirits found in many other religious traditions. Other roles of angels include protecting and guiding human beings, and carrying out God's tasks.