Wu

1.
A Chinese term corresponding to the Sanskrit bodhi, meaning 'enlightenment' or 'awakening'. The same character forms the root of the Japanese term satori.

2.
A Chinese term meaning 'non-being' or 'to lack'. In the earliest attempts by Chinese Buddhists to understand Indian Buddhist thought and translate texts, wu was used to mean 'emptiness' (sunyata), but was later supplanted by the word k'ung. The word wu has been retained to denote the absence of all distinguishing characteristics that would separate phenomena from each other in an ultimate way; such as, it is the negation of all dualities. In this regard, it appears in the compound pen wu, or 'original nonbeing', in contraposition to miayo wu, or 'marvelous being', which complements it by affirming the real existence of separate things within the matrix of Dependent Origination (pratitya-samut-pada). In the famous riddle (koan) 'Chao-chou's dog', a monk asked Chao-chou Ts'ung-shen whether or not a dog had Buddha-nature, to which the master replied, 'wu', meaning it has not. This word 'wu', then, became the 'critical phrase' (Chinese, hua-t'ou), and the object of meditation when working with this koan.

---Dictionary of Buddhism, by Damien Keown---