The Spanish word 'adicional' (meaning 'additional') comes from the Latin verb 'addere' meaning 'to add'. From this verb, Latin developed the noun 'additio' meaning 'addition'. This word evolved into Spanish 'adición' (meaning 'addition'), and when combined with the Spanish adjective suffix '-al', it formed 'adicional'.

The development from Latin to Spanish shows typical sound changes, where the double 'd' in Latin 'additio' simplified to a single 'd' in Spanish, and the 't' softened to 'c' before the 'i'.

Some related and simpler Spanish words include 'adición' (addition) and 'añadir' (to add). While 'adicional' is an adjective describing something extra or additional, 'adición' is the noun form referring to the act of adding something. These words share the same Latin root 'addere' and maintain closely related meanings in modern Spanish.

English speakers can easily connect 'adicional' to several familiar English words like 'addition', 'additional', and 'add'. All these words come from the same Latin root 'addere'. The similarity in both form and meaning between Spanish 'adicional' and English 'additional' makes this word particularly easy for English speakers to remember and use.