'25th' is formed in Modern English by combining the Arabic numeral '25' with the English ordinal suffix 'th'. The suffix 'th' itself traces to Old English ordinal endings (written with the letter þ, pronounced like th).

'twenty-fifth' developed from the compound of 'twenty' + 'fifth'. 'Twenty' comes from Old English 'twentig' (Middle English 'twentie'), and 'five' comes from Old English 'fīf'. Over time the spoken compound 'twenty fifth' became written as 'twenty-fifth' and may be shortened in numeric form to '25th'.

Initially the components signified 'twenty + five' as a cardinal composition; with the addition of the ordinal suffix the expression came to denote the position 'the one that is number 25' in a sequence, which is its current use.