The term 'constant-pressure' is formed from the English words 'constant' meaning 'unchanging' and 'pressure' meaning 'force exerted per unit area.'

'Constant' comes from Latin 'constans' (standing firm), and 'pressure' from Latin 'pressura' (act of pressing). The compound 'constant-pressure' emerged in scientific English to describe processes where pressure does not change.

Initially, 'constant-pressure' referred specifically to scientific or engineering contexts where pressure remains unchanged, and this meaning has remained consistent.