Nível de Energia é uma leitura do Ki atual de uma pessoa.
The actual term "Power Level" usually refers to readings produced by scouters, which are sensory devices used by Frieza's crew that detect a person's ki. The reading is converted into numbers of an unspecified unit, and the resulting number is the "Power Level", or an estimation of that person's physical capability.
It is first introduced in Dragon Ball, where a young Goku learns to sense ki after drinking the Ultra Divine Water, although the Z Fighters are able to detect power levels via the ki sensing ability eventually.