Qì=Pinyin; Ch'i=Wades/Giles; Traditional Chinese=氣; Simplified Chinese=气. A literal translation of "qi" is "breath", "air", or "gas".
Merriam-Webster: In Chinese philosophy, the ethereal substance of which everything is composed. Early Daoist philosophers and alchemists regarded it as a vital force associated with breath and bodily fluids and sought to control its movement within the body in order to achieve longevity and spiritual power. Manipulation of qi is central to Chinese meditation, medicine, and martial arts. In the 10th–13th centuries Neo-Confucianism regarded qi as emanating from the Great Ultimate by way of li, the ordering principle of the universe, transformed into the elements through yin and yang. (1)
Cambridge Advanced Learner's Dictionary: "Chi". noun. In some systems of Chinese medicine and exercise, the most important energy that a person has. (2)
Oxford Dictionaries: noun. The circulating life force whose existence and properties are the basis of much Chinese philosophy and medicine. Origin: from Chinese qì, literally 'air, breath' (3)
TheFreedDictionary.com: noun Qi - the circulating life energy that in Chinese philosophy is thought to be inherent in all things; in traditional Chinese medicine the balance of negative and positive forms in the body is believed to be essential for good health. (4)
Urban Dictionary: An ancient magical Chinese superpower force used as a plot device in kung-fu movies when the writers are having a slow day.