Anaerobic is a technical word which literally means without air (where "air" is generally used to mean oxygen), as opposed to aerobic.

In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen. An anaerobic adhesive is a bonding agent that doesn't cure in the presence of air.

Anaerobic exercise is exercise intense enough to trigger anaerobic metabolism. It is used by athletes in non-endurance sports to promote strength, speed and power and by body builders to build muscle mass. Muscles trained using anaerobic exercise develop differently compared to aerobic exercise, leading to greater performance in short duration, high intensity activities, which last from mere seconds up to about 2 minutes. Any activity after about two minutes will have a large aerobic metabolic component.

Anaerobic metabolism, or anaerobic energy expenditure, is a natural part of whole-body metabolic energy expenditure. Fast twitch skeletal muscle (as compared to slow twitch muscle) operates using anaerobic metabolic systems, such that any recruitment of fast twitch muscle fibers will lead to increased anaerobic energy expenditure. Intense exercise lasting upwards of about four minutes (e.g., a mile race) may still have a considerable anaerobic energy expenditure component. Anaerobic energy expenditure is difficult to accurately quantify, although several reasonable methods to estimate the anaerobic component to exercise are available.
In contrast, aerobic exercise includes lower intensity activities performed for longer periods of time. Activities such as walking, running (including the training known as an interval workout), swimming, and cycling require a great deal of oxygen to generate the energy needed for prolonged exercise (i.e., aerobic energy expenditure). In sports which require repeated short bursts of exercise however, it is the anaerobic system that enables muscles to recover for the next burst. Therefore training for many sports demands that both energy producing systems be developed.

There are two types of anaerobic energy systems: 1) the high energy phosphates, ATP adenosine triphosphate and CP creatine phosphate and, 2) anaerobic glycolysis. The high energy phosphates are stored in very limited quantities within muscle cells. Anaerobic glycolysis exclusively uses glucose (and glycogen) as a fuel in the absence of oxygen or more specifically, when ATP is needed at rates that exceed those provided by aerobic metabolism; the consequence of rapid glucose breakdown is the formation of lactic acid (more appropriately, lactate at biological pH levels). Physical activities that last up to about thirty seconds rely primarily on the former, ATP-PC phosphagen, system. Beyond this time both aerobic and anaerobic glycolytic metabolic systems begin to predominate. The by-product of anaerobic glycolysis, lactate, has traditionally thought to be detrimental to muscle function. However, this appears likely only when lactate levels are very high. In reality, many changes occur within and around muscle cells during intense exercise that can lead to fatigue, with elevated lactate levels being only one (fatigue, that is muscular failure, is a complex subject). Elevated muscle and blood lactate concentrations are a natural consequence of physical exertion, regardless of what form it takes: easy, moderate, hard or severe. The effectiveness of anaerobic activity can be improved through training.
Lactate threshold (LIP or lactate inflection point)

The lactate threshold (LT) is the exercise intensity at which lactic acid starts to accumulate in the blood stream. (This is not strictly true, as 'lactic acid' per se does not exist at the pH-levels encountered in the body. Its anion, the lactate molecule, accumulates in the blood—hence its usage in 'onset of blood lactate accumulation' (OBLA) is 'lactate' and not 'lactic acid.' The reason for the acidification of the blood at high exercise intensities is two-fold: the high rates of ATP hydrolysis in the muscle release hydrogen ions, as they are co-transported out of the muscle into the blood via the MCT—monocarboxylate transporter, and also bicarbonate stores in the blood begin to be used up.) This happens when it is produced faster than it can be removed (metabolized). This point is sometimes referred to as the anaerobic threshold (AT), or the onset of blood lactate accumulation (OBLA). When exercising below the LT intensity any lactate produced by the muscles is removed by the body without it building up. The lactate threshold is a useful measure for deciding exercise intensity for training and racing in endurance sports (e.g. long distance running, cycling, rowing, swimming, motocross, and cross country skiing), and can be increased greatly with training. The anaerobic threshold is considered to be somewhere between 90% and 95% of your maximum heart rate. Interval training takes advantage of the body being able to temporarily exceed the lactate threshold, and then recover (reduce blood-lactate) while operating below the threshold and while still doing physical activity. Fartlek and interval training are similar, the main difference being the relative intensities of the exercise, best illustrated in a real-world example: Fartlek training would involve constantly running, for a period time running just above the lactate threshold, and then running at just below it, while interval training would be running quite high above the threshold, but then slowing to a walk or slow jog during the rest periods. Interval training can take the form of many different types of exercise and should closely replicate the movements found in the sport.(3)

Accurately measuring the lactate threshold involves taking blood samples (normally a pinprick to the finger, earlobe or thumb) during a ramp test where the exercise intensity is progressively increased. Measuring the threshold can also be performed non-invasively using gas-exchange (Respiratory quotient) methods, which requires a metabolic cart to measure air inspired and expired.

Although the lactate threshold is defined as the point when lactic acid starts to accumulate, some testers approximate this by using the point at which lactate reaches a concentration of 4 mM (at rest it is around 1 mM).