A meniscus is the curved top surface of a column of liquid. The meniscus can be either concave or convex.
A concave meniscus occurs when molecules are more attracted to the container than to each other. The liquid appears to ‘climb’ the sides of the container. Most liquids possess a concave meniscus.
Figure: Concave and convex meniscus
A convex meniscus is the opposite of a concave meniscus. An example of a molecule with a convex meniscus is mercury. A flat meniscus is possible when the container is made of certain types of plastic or material that the contained substance does not stick to.