A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Select the first letter of the word from the list above to jump to appropriate section of the glossary. If the term you are looking for starts with a digit or symbol, choose the '#' link. - O - opposition The position of a superior planet when it is closest to the Earth. (At this point it is 180 degrees from the Sun's position.) orbit The path of a celestial object around another object. Orion Definition. Orion Nebula Visible with the naked eye, this nebula is located in the constellation of Orion, and is the largest complex of gas and dust known in the Milky Way. Back to Top