Let us consider, a glass plate is dipped vertically inside a liquid and the liquid wets the plate, as in case of water, alcohol, etc. The level of liquid rises up a little along the side of the plate as shown in the adjoining Fig.1. On the other hand, when a glass plate is dipped vertically inside a liquid which does not wet the wall of the plate, as in case of mercury, the level of the liquid is depressed a little along the region of contact, as shown in the Fig.2.

So when a solid is dipped into a liquid, the liquid surface near the wall of contact is not horizontal- it is either a little bit elevated or depressed, depending on the nature of the liquid and the solid.

So the angle of contact is defined as the angle which the tangent at the point of contact of the liquid surface and solid surface makes with the immersed part of the solid which partly and vertically dipped within the liquid. It is denoted by \( \theta \)