The Spanish word 'plana' comes directly from the Latin word 'plana', which was the feminine form of the adjective meaning 'flat' or 'level'. This word has maintained its original form and meaning remarkably well as it transitioned from Latin to Spanish.

Several common Spanish words share this root, including 'plano' (flat, plain), 'planear' (to glide, to plan), and 'planicie' (plain, flat land). All these words carry the basic concept of flatness or levelness in their meanings.

English speakers might recognize this root in words like 'plane' (as in airplane or a flat surface), 'plain' (flat land or simple), and 'plan' (a flat drawing or scheme). The concept of flatness is preserved in these English cognates, though some have developed extended meanings over time.