The meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension

Surface tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

. It can be either convex or concave

Convex set

In Euclidean space, an object is convex if for every pair of points within the object, every point on the straight line segment that joins them is also within the object...

. A convex meniscus occurs when the molecules have a stronger attraction to each other (cohesion

Cohesion (chemistry)

Cohesion or cohesive attraction or cohesive force is the action or property of like molecules sticking together, being mutually attractive...

) than to the material of the container (adhesion

Adhesion

Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....

). This may be seen between mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 and glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 in barometers and thermometers. One can over-fill a glass with mercury and produce a convex meniscus that rises above the top of the glass.