Aura

Aura is the manifestation of one's soul and can be used for a wide range of abilities. The specific abilities and their strengths differ from individual to individual and are dependent on a number of factors, such as experience, training and innate skill. It is predominantly used for protection from harm. Some people develop an active power called a Semblance.
  Aura is an ability that all those with a soul can use, at least with enough practice. Because Aura is the manifestation of the soul, it can appear very differently depending on who is using it. It is mentioned that anything with a soul has an Aura, including Humans, Faunus, and animals; even synthetic people with souls, can generate an Aura. The Creatures of Grimm, being soulless, are the only known form of life that does not have Aura. Skilled users of Aura can create their own barriers or even increase their own abilities in some way. Weapons and armor can also act as a conduit for Aura, allowing for an even wider range of offensive and defensive capabilities. This even applies to nontraditional "weapons", such as a loaf of bread or a leek.
  It has been shown that the strength of an Aura degrades with continual use. This limits the applications of Aura in combat as, with time or heavy use, a person's Aura will decay to the point where they are left weakened or even incapacitated. Aura being completely depleted is visually represented as a colored sheen glowing along the length of a person's body or with colored particles flying off the body, indicating that they can now be physically injured. Aura flickering means it is close to being depleted. Aura can be recharged and the time it requires to recharge is varies between each person. Aura also can vary between individuals in terms of innate quantity of energy that can be used. Aura is a necessary energy source for all Huntsmen, being a powerful ally in battle. It is also required in order to trigger the use of Dust.
  The former Kingdom of Atlas was performing serious scientific research into the nature and composition of Aura. Their research yielded experimental technology that was capable of "capturing" Aura from a person and transferring it to something or someone else. When the Penny project was singled out for production, it's creator, Pietro Polendina, was able to splice off part of his Aura and give it to his daughter Penny, making her the first synthetic person with an Aura.
  The existence of this technology was highly classified, but many of those who did know of it viewed it as ethically immoral and wrong.