Ki 「氣」 is a natural energy given off by people and objects when they move. It exists separate from mana.

Japanese
氣

Rōmaji

Ki

Official English
Ki (VIZ, Funimation)

Overview

With proper training, ki-sensing can be used to predict attacks and others' movements, including natural objects and events found in nature. It can also be used to detect whether someone is lying.

Each individual's ki is unique, and it is rare for different individuals' ki to be the same.

Trivia
Ki, or , is a life energy in Chinese and Japanese culture.