Sky Void in Chinese / Japanese...

空 is a single character that means empty, void, hollow, vacant, vacuum, blank, nonexistent, vacuity, voidness, emptiness, non-existence, immateriality, unreality, the false or illusory nature of all existence, and being unreal.

In the Buddhist context, this relates to the doctrine that all phenomena and the ego have no reality but are composed of a certain number of skandhas or elements, which disintegrate. The void, the sky, space. The universal, the absolute, complete abstraction without relativity. The doctrine further explains that all things are compounds, or unstable organisms, possessing no self-essence, i.e. are dependent, or caused, come into existence only to perish. The underlying reality, the principle of eternal relativity, or non-infinity, i.e. śūnya, permeates all phenomena making possible their evolution.

From Sanskrit and/or Pali, this is the translation to Chinese and Japanese of the title śūnya or śūnyatā.

In Japanese, when pronounced as “ron” (sounds like “roan”) this can be a given name. It should be noted that this Kanji has about 5 different possible pronunciations in Japanese: kuu, kara, sora, ron, and uro. 空 is also an element in the Japanese version of the five elements.

虛空 means void, hollow, empty, space, sky, atmosphere, heaven, or ether.

虛空 is the Chinese and Japanese version of the Sanskrit word ākāśa (or akasa / akash) which, beyond the sky or space meaning can be the immaterial universe behind all phenomena in the Buddhist context.

虛空 means empty space, empty sky, or void.

In the Buddhist context, it can mean “emptiness of the material world.” This can also be used as an adjective to modify other words with a meaning of unreal or insubstantial.

天空 means sky in most contexts but it can also refer to air, space, the heavens, or ether.

地水火風空 is the specifically-Japanese version of the five elements.

This is a little different than the ancient or original Chinese version.

The elements are written in this order:
1. Earth / Terra / Ground
2. Water
3. Fire
4. Wind / Air
5. Sky / Emptiness / Void / Ether

Note: This set of Kanji can also be romanized as “ji sui ka fuu kuu,” “jisuikafuukuu,” or “jisuikafuku.”

These can also be written in the order 地火風水空 (chi ka sui fuu kuu). Let me know when you place your order if you want the Kanji to be in this character order.

空無 is “nothingness” in a Buddhist context.

The first character means empty but can also mean air or sky (air and sky have no form).

The second character means have not, no, none, not, or to lack.

Together these characters reinforce each other into a word that means “absolute nothingness.”

I know this is a term used in Buddhism, but I have not yet figured out the context in which it is used. I suppose it can be the fact that Buddhists believe that the world is a non-real illusion, or perhaps it's about visualizing yourself as “nothing” and therefore leaving behind your desire and worldliness.
Buddhist concepts and titles often have this element of ambiguity or, rather, “mystery.” Therefore, such ideas can have different meanings to different people, and that's okay. If you don't get it right in this lifetime, there will be plenty more lifetimes to master it (whatever “it” is, and if “it” really exists at all).

Soothill defines this as “Unreality, or immateriality, of things, which is defined as nothing existing of independent or self-contained nature.”

虚実 is a Japanese word that means “falsehood [and] truth” or “fiction [and] fact.”

This concept is used in warfare, gameplay, and martial arts strategies. 虚実 can be a strategy of real and/or deceptive moves. This gets to some Sun Tzu Art of War stuff where in warfare a strategic move is either a real and serious move or it is a deceptive blow.

Let's explore each character in more depth:

虚 was originally written 虛 (there is a very subtle difference in the strokes at the bottom of the character) and means unpreparedness, falsehood, emptiness, void, abstract theory, empty or unoccupied, diffident or timid, false, humble or modest, virtual, or in vain.
In the Buddhist context, 虛 represents the Pali/Sanskrit word “śūnya,” meaning empty, vacant, unreal, unsubstantial, untrue, space, humble, or in vain.
In ancient Eastern/Chinese astronomy, 虛 represents the “Emptiness” constellation (one of the 28 mansions in the sky).

実 was originally 實 in Chinese (they currently write it as 实 in Simplified Chinese) with the meaning, truth, reality, sincerity, honesty, fidelity, and substance.
The Buddhist context is similar, adding real, true, honest, really, solid, definitely, sincere, solid, fixed, full, to fill, fruit, kernel, verily, in fact, the supreme fact, or ultimate reality to the definition.