精 Jīng (pinyin) or ching1 (Wade-Giles) is the Chinese word for "essence", more specifically the kidney essence or semen. According to traditional Chinese medicine, jing is stored in the kidneys and is the most dense physical matter within the body (as opposed to the shen which is the most volatile). It is said to be the material basis for the physical body and is yin in nature, which means it nourishes, fuels, and cools the body. As such it is an important concept in the nei chia school of the Chinese martial arts. Jing is also believed by some to be the carrier of our heritage (e.g. DNA); in the man the semen and in the woman the menstrual blood are believed to be part of jing. Theoretically, jing is consumed continuously in life; by everyday stress, illness, substance abuse, sexual intemperance, etc. Many qigong related disciplines are devoted to replenishing "lost" jin.
Jīng 精 is often confused by people who don't speak Chinese with the related concept of jėn 勁 or power. See nei jin.