(1) Not requiring, or capable of occurring, in the absence of air or free oxygen.
(2) Caused by, or relating to, the lack of molecular oxygen.
Anaerobic may be used to describe an organism, a cell, a process or a mechanism that can function without air (i.e. air to generally mean oxygen). This is in contrast to the term aerobic, which means requiring air or free oxygen.
Word origin: from French anaérobie, coined in 1863 by Louis Pasteur, from Greek an- (“without”) + aer (“air”) + bios (“life”).