皮革 (pí gé) is a compound noun that specifically means 'leather' — the processed material from animal hides. It is more precise and formal than the single character 皮 (pí), which can also mean 'skin' or 'fur' depending on context. Use 皮革 when referring to the material itself (e.g., a leather coat), not to human or animal skin (that's 皮肤 pí fū).