The triple point of any substance is that temperature and pressure at which the material can coexist in all
three phases (solid, liquid and gas) in equilibrium.
Specifically the triple point of water is 273.16 K at 611.2 Pa
Answered by: Ryan Hitch, MPhys Physics, University of Hull
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '