The word "天空" is formed through the semantic combination of two characters: "天", meaning "sky" or "heaven", and "空", meaning "empty" or "void". Together, they create a compound where "天" establishes the primary concept of the celestial realm, while "空" describes its characteristic expanse and openness, thus collectively evoking the image of the vast, open sky above. This formation relies on the conceptual blending of meanings to produce a new, unified term that captures the essence of the sky's boundless nature.