In the world of Hunter × Hunter, aura is an energy produced by all living bodies vital for survival. The loss of one's aura is the same as losing the energy keeping the body alive, resulting in death. Aura from all parts of the body has a tendency to flow together, producing one mass of energy. This happens without the individual's awareness, and will simply leak out of their body slowly. The pores or points on the body where aura flows out from are called aura nodes.

A Nen user trains to manually open and close their aura nodes so that they can control the flow of their aura. One can learn this process gradually, learning to control the aura nodes through meditation, or they can receive an influx of aura from an experienced user that forces these nodes open, forcing the new user to learn to control the flow or risk suffering severe exhaustion and/or death. The second method is generally frowned upon as it very dangerous (although far quicker). Once one has opened their aura nodes, they will be able to see their aura as a shroud surrounding the body. Some people are able to 'discover' and learn to manipulate their aura on their own, without any teaching or guidance from another Nen user.

Although the production of aura is unconscious and constant by all living beings, it is more than just 'leaky energy'. Aura carries with it the desires and emotions of the one who deploys it, which is what allows for Nen to have incredible versatility to those who develop their skill at using it. Nen is also heavily influenced by mental condition and state. A basic application of this phenomenon is that one can channel their aggression or hatred into their Nen and deploy it towards another person. The other person will then be able to feel that aggression as if it were physically palpable, and if unable to keep it from their own body by deploying their own Nen, may be physically harmed by it.

Nen responds to the goals and strengths of the desire of its user. As a result, a user can make a commitment that results in an increase in their abilities. If a user, for example, damages their own body in such a way that an individual skill seems more imposing when used, their Nen will become correspondingly more powerful. It is also possible to increase the strength of an individual skill by imposing limits on it. For example, if a user consciously decides something along the lines of "I will only use this skill on Thursdays," or "I will only use this skill against short people," and manage to abide by the rule, that particular skill will become stronger.
Theoretically, the more restrictive the condition, the greater the enhancement in power. It is also implied that restrictions which carry great meaning, or emotional value also bring about a greater benefit. Furthermore, restrictions which contain some sort of 'punishment' (i.e. "I will die should I break this rule") will strengthen the ability more.

When one has developed use of their nen, they become sensitive to the presence of other beings by their deployment of nen. Since every living being creates nen subconsciously, it is a useful skill for those tracking or hunting (both animate and inanimate things). It is also useful in combat, because one can judge the location and relative strength of opponents by the output of their nen. To counter this, techniques that minimize nen output like Zetsu or In are used.

Nen is like any other skill, in that there are those who will learn it faster than others. There are also individuals that can use facilities of nen without having any formal training. This can be anything from being able to use the basic techniques of nen without even realizing it, or unconsciously developing a unique nen ability that can be used without the user really understanding how or why they are able to do so.