The triple point is a Thermodynamic Phenomenon, It is a point where an element exists in Solid, Liquid and Gaseous form and in Thermodynamic Equilibrium. For example, the triple point of Acetylene occurs at -80.7° Celsius.
Triple Point of Water
As an addendum to the above definition, it becomes imperative to state here that, the Triple point is created with the perfect synergy of temperature and pressure. The best way to understand this is by observing the triple point of water.
Water reaches its triple point at exactly 0.01 Degree Celcius and at a pressure of 611.73 pascals. Now at this point, the water exists as vapor, liquid and solid ice at the same time. A small change in temperature and we would be able to convert it into any other form instantly.
In certain cases like in case of Helium, a triple point may also occur involving more than one solid phase. This happens with things which have different polymorphs. For example, Helium 4 exhibits two different liquid states at the triple point. This is represented by Lambda Point.
Lambda point is the temperature at which normal helium makes a transition into super helium.