Anaerobic Respiration (respiration without oxygen) is the opposite of aerobic respiration (respiration with oxygen). Essentially, when a cell needs to break down a fuel source and produce ATP energy, it must initiate the process with either oxygen (aerobic) or another energy source (anaerobic). The cell takes in the fuel and processes it with another energy source, such as glucose (sugar), which allows the cell to continue living in the absence of oxygen. Most cells in the human body use aerobic respiration, taking their oxygen from the blood and returning carbon dioxide in waste. Some bacteria operate without oxygen, and the human muscle enters a state of anaerobic respiration after it exhausts the available oxygen and is unable to receive more quickly, replacing oxygen with lactic acid.
When the cellular respiration takes place in absence of oxygen then it is called as anaerobic respiration.
Respiration are of two types they are aerobic and anaerobic respiration.
Here respiration doesn't mean taking air via nostrils, here respiration means its a cellular process where glucose is broken down to produce energy which is inturn utilised for cellular activity. If such cellular respiration takes place in absence of oxygen then it is called of anaerobic respiration and it takes place in presence of oxygen then it is called as aerobic respiration.