'mid-20th-century' is a compound formed from 'mid' + '20th' (ordinal of '20') + 'century'. 'Mid' originates from Old English 'mid', where 'mid' meant 'middle'; 'century' originates from Latin 'centuria' (from 'centum'), where 'centum' meant 'one hundred'. The ordinal element '20th' is formed from the cardinal 'twenty' plus the ordinal suffix '-th' (from Old English/Old Norse usage).

'mid' (Old English 'mid') combined with ordinal-number + 'century' (via Latin 'centuria' and Old French 'centurie') to produce phrases like 'mid-century' in earlier modern English; specifying the century (e.g. 'mid-20th century' or hyphenated 'mid-20th-century' as a modifier) became common in 19th–20th-century usage as writers referred to particular historical periods.

Initially, 'mid-century' simply meant 'middle of a given century'; over time the compound 'mid-20th-century' came to be used specifically to evoke cultural, artistic, social, or technological characteristics associated with the mid 1900s, and is now commonly used as a descriptive label for that period.