Meniscus, plural: menisci, from the Greek for "crescent", is a curve in the surface of a liquid and is produced in response to the surface of the container or another object. It can be either concave or convex. A convex meniscus occurs when the molecules of the liquid repel the molecules of the container or object. This may be seen between mercury and glass in barometers. Conversely, a concave meniscus occurs when the molecules of the liquid attract those of the container. This can be seen between water and glass. Surface tension acts on concave menisci to pull the liquid up, and on convex menisci to pull the liquid down. This phenomenon is important in transpirational pull in plants.
When reading a scale on the side of a container filled with liquid, the meniscus must be taken into account in order to obtain a precise measurement. The measurement is taken with the meniscus at eye level to eliminate parallax error, and at the central point of the curve of said meniscus, i.e. the top of the meniscus, in the unusual case of a liquid like mercury, or more usually, the bottom of the meniscus in water and most other liquids. The reason for this is to attain maximum precision in the measurements so that if the measurements are accurate they will then be valid.