The aerobic threshold is a term sometimes used by sports coaches and trainers to describe a level of exercise somewhat below the anaerobic threshold. It, however, is not a defined physiological term.
The AeT is sometimes defined as the exercise intensity at which anaerobic energy pathways start to operate and where blood lactate reaches a concentration

Concentration

In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...

 of 2 mmol

Mole (unit)

The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

/litre (at rest it is around 1). This tends to be at a heartrate of approximately 20-40 bpm less than the anaerobic threshold and correlates with about 65% of the maximum heart rate. The anaerobic energy system does not utilise oxygen to create Adenosine triphosphate (ATP)and uses glycogen/glucose. Lactic acid is the by-product of creating ATP for working muscles.