Chemosynthesis is a process certain organisms use to obtain energy for the production of food, akin to photosynthesis, but without the use of sunlight. The energy comes from the oxidization of inorganic chemicals that the organisms find in their environment. The process occurs in many bacteria, and in another group of organisms known as archaea. The life forms that use this method to obtain energy are found in a variety of environments, including soil, the intestines of mammals, petroleum deposits, and in extreme conditions, such as around hydrothermal vents on the ocean floor. They are adapted to circumstances which may have been commonplace billions of years ago, leading some scientists to theorize that they may be direct descendants of the earliest life on Earth.
In short- the synthesis of organic compounds by bacteria or other living organisms using energy derived from reactions involving inorganic chemicals, typically in the absence of sunlight.