Three Emptinesses

The term "Empty" or "Emptiness" is often used in Internal Martial Arts (内家武術) translations, in particular those that are somewhat older. In fact, whenever reading a text that mentions "emptiness" in a western language one should be aware that it can point to three different notions.

空

The first translation is what I would call "literal" emptiness, the notion that something has been emptied, is vacant, unoccupied. This term is kōng (空), and it describes a wide range of things: the outer space in astronomy (太空), the vacuum (真空), a cavity (空洞), a vacancy (空缺) or a "liberty" in the game of go.

In the internal martial arts texts I've seen, this term is used mostly linked to physical postures, like the necessity of emptying the chest. For example in "Cover" (扣) from the "Xingyi Eight Characters Secret" (形意八字訣) by Jiāng Róngqiáo (薑容樵):

一。兩肩要扣，則前胸空闊，氣力到肘。

"Both shoulders should be like covers. The chest is empty and broad. The qi strength arrives in the elbows."

虚

A second translation is mostly in Tai Chi Chuan (太極拳), and it relates to the way movement is made in Tai Chi. Earlier translations would talk about "full" (實) and "empty" (虚) for the weight for example. I think a better translation for this "fullness" and "emptiness" is given by Yang Jwing Ming (楊俊敏): "substantial" (實) and "insubstantial" (虚). The idea of emptiness here is an idea of void, often in what in the West would be seen as a negative sense: a false reputation (虚名), vanity (虚荣心) or fictitious or theoretical (虚拟). In the Tai Chi Chuan texts, the term is quite central to the practice, for example in the The Ten Important Points of Tai Chi Chuan (太極拳十要) by Yáng Chéngfǔ (楊澄甫):

太極拳術以分虛實為第一義

"The discrimination of insubstantial and substantial is the most important content in Taijiquan. "

無
極

Although much more rarely, the third term that is sometimes translated as "emptiness" or a related term is Wú​jí (無極). It is a term related to chinese cosmogony, as it refers to the potential state the universe was before the "Ten Thousand Beings" appeared (萬物). In internal martial arts, Wú​jí is used to denote the initial undifferentiated state, before a form is started. In some old translations or internal martial arts books, we would see "emptiness" or "empty oneself" related to that specific point.

The term has been introduced in the Internal Martial Arts universe by Sūn Lùtáng (孫祿堂) and Jiāng Róngqiáo (薑容樵), in texts like "The Song of Wújí" (無極歌) by Jiāng Róngqiáo (薑容樵) and "The Study of Wuji in the Sword" (無極劍學) by Sūn Lùtáng.