'completion' originates from Latin, specifically the word 'completio', where 'com-' meant 'together' and 'plēre' (from which 'plēre' is related) meant 'to fill'.

'completion' changed from Medieval/Latin 'completio' and Old French 'completion' into Middle English forms such as 'completioun' and eventually became the modern English word 'completion'.

Initially, it meant 'a filling up' or 'the state of being filled', but over time it evolved into its current primary meaning of 'the act or process of finishing' or 'the finished state'.