天 (tiān): Sky, heaven. A simple character often pictured as a line representing the sky above a person (大).
壤 (rǎng): Earth, soil. The left side (土) is the radical for “earth,” and the right side provides the sound.
之 (zhī): A classical grammatical particle that connects the words before and after it, similar to the English “'s” or “of.”
别 (bié): Difference, distinction.
Putting them together, 天壤之别 (tiān rǎng zhī bié) literally translates to “The difference of heaven and earth.” The imagery is stark and immediate: the highest point imaginable (heaven) versus the ground beneath our feet (earth). The distance and difference between them is immeasurable, which is precisely the feeling this idiom conveys.