Vapour pressure (vapour head). Term denoting the pressure which a vapour in equilibrium with its (liquid or solid) base inside an enclosed vessel exerts on the enclosing walls. Vapour pressure depends solely on the temperature. If the vapour pressure lies close to the pressure to be sealed, there is a risk of vaporization in the sealing gap. The graph shows the vapour pressure curves of a number of media often needing to be sealed.