The Spanish word 'piano' comes from the Italian word 'pianoforte', which literally means 'soft-loud'. This name originated from the instrument's ability to play both soft and loud notes, unlike its predecessors. The 'piano' part comes from the Italian word 'piano' meaning 'soft' or 'gentle', which itself derives from Latin 'planus' meaning 'flat, level, clear'. The instrument became known simply as 'piano', dropping the '-forte' part over time.

The Spanish word 'plano' (meaning 'flat' or 'plane') is a related word, also coming from Latin 'planus'. While 'piano' evolved to refer to the musical instrument through Italian, 'plano' maintained the original spatial meaning of the Latin word.

Several English words share the same Latin root 'planus': 'plane' (as in a flat surface or airplane), 'plain' (simple, clear, or a flat area of land), and 'plan' (originally referring to a flat drawing). The English word 'piano' was also borrowed from Italian, just like the Spanish word. Additionally, we have words like 'planar' (relating to a flat surface) and 'explanation' (literally, making something flat or clear) from the same Latin root.