'mediaeval' originates from Medieval Latin, specifically the phrase 'medium aevum', where 'medium' meant 'middle' and 'aevum' meant 'age'.

'mediaeval' changed from Medieval Latin 'mediaevalis' (from 'medium aevum'), passed into Late Latin and Old/Middle French forms, and entered Middle English as 'mediaeval' or variant spellings before the modern English forms 'mediaeval'/'medieval'.

Initially it referred literally to the 'middle age' ('medium aevum'), and over time it came to mean 'relating to the Middle Ages' (used as an adjective) and to describe styles, institutions, and ideas from that period.