Both ching and ch'l (spirit, breath) are terms used by Taoist philosophical alchemists seeking immortality. Ching, in this context, is the product of properly developed ch'i. It can also mean semen or, in another sense, and according to a different Chinese character, "muscular force." Sometimes they are contrasted, as between the ch'i of the internal kung-fu systems and the ching of the external styles, but more often ching is thought to be of a nature directly related to ch'i.
Further reading: Taolst Yoga, Lu K'uan Yu, 1970; The Yellow Emperor's Classic of Internal Medicine, liza Veith, 1972.