Aether (or æther) is an obsolete spelling of the noun with various definitions relating to air, chemical compounds, and nonexistent clear elements filling outer space. The word is usually spelled ether in all main varieties of modern English. Aether still appears as a poetic affectation, especially in reference to the word’s ancient Greek and Latin senses, but ether is preferred by a large margin in most types of writing.
The æ in æther is a Latin-derived letter that represents different sounds in different languages (and it still appears in some modern alphabets). The letter was preserved in numerous Latin words brought to English between the 16th and 19th centuries, though later in this period æ was usually rendered ae, as it usually is today. Many of these words, like ether, have dropped the a over the last century or so, so that, for instance, encycolopaedia is now encyclopedia, anaesthesia has become anesthesia, and aeon has become eon. Some of them have dropped the a in American English but retain it elsewhere—for example, pediatric/paediatric, anemia/anaemia, and feces/faeces. Others retain the a everywhere—for example, aesthetic, aegis, and paean.