contact angle

https://dx.doi.org/10.1351/goldbook.C01290
When a liquid does not spread on a substrate (usually a solid), a contact angle(θ) is formed which is defined as the angle between two of the interfaces at the three-phase line of contact. It must always be stated which interfaces are used to define θ. It is often necessary to distinguish between the 'advancing contact angle' (θ a), the 'receding contact angle' (θ r) and the 'equilibrium contact angle' (θ e). When θ r ≠ θ a the system is said to exhibit contact anglehysteresis
Source:
PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 598