'term-certain' is a compound formed from 'term' + 'certain'. 'term' originates from Latin, specifically the word 'terminus' meaning 'boundary, limit'. 'certain' originates from Latin, specifically the word 'certus', meaning 'sure' or 'settled'.

'term' passed into English via Old French 'term(e)' and Middle English 'terme'; 'certain' passed into English via Old French 'certain' from Latin 'certus'. The compound usage as a fixed legal phrase (term certain/term-certain) developed in legal English to describe a definite period.

Originally 'term' referred to a boundary or fixed limit and 'certain' meant 'sure/settled'; combined, the phrase came to mean 'a limit that is fixed or settled' and is now used to mean a definite, fixed duration.