The Spanish word 'tono' comes from Latin 'tonus', meaning 'tone, sound, or accent'. The Latin word was itself borrowed from Ancient Greek 'τόνος' (tonos), which meant 'tension' or 'tone'. The Greek word originally referred to the tension of a stretched string, which produces different musical tones when plucked - a meaning that evolved to refer to sound and pitch more generally.

Several common Spanish words are related to 'tono', including 'entonar' (to tune or intone), 'tónico' (tonic), and 'tonicidad' (tonicity). These words all share the basic concept of tone or tension from their common Greek root.

English speakers will recognize many cognates from this same Greek root, including 'tone', 'tonic', 'monotone', and 'baritone'. The connection is easy to remember since both Spanish 'tono' and English 'tone' have nearly identical meaning and pronunciation. Even the English word 'tense' (as in tension) is distantly related, reflecting the original Greek meaning of physical stretching or tightness.