Meniscus
The meniscus is the curved upper surface of a liquid in a container; water volume is read at the bottom of the curve at eye level.
Definition
The meniscus is the curve you see at the top of water in a graduated cylinder: the water climbs the sides a little, so the middle dips lower than the edges. It forms because of the balance between the liquid's attraction to the container walls (adhesion) and to itself (cohesion); water adheres to glass and forms a concave meniscus read at the bottom of the curve, while mercury forms a convex one read at the top. Physically it is the curved liquid-air interface set by surface tension and the contact angle, with an angle below $90^\circ$ giving a concave meniscus and capillary rise. Volumetric glassware is calibrated for the bottom of a concave meniscus read at eye level, so parallax and a greasy wall that alters the contact angle are both systematic errors in volume.
Example
Looking at water in a cylinder with your eye level to the surface, you see a small U shape, and you read the number at the bottom of the U rather than at the edges where the water touches the glass. In a $100\ \text{mL}$ cylinder the edges touch at about $24.0$ while the bottom of the curve sits at $23.4$, so the correct reading is $23.4\ \text{mL}$ and reading the edge would overstate the volume by $0.6\ \text{mL}$, about $2.5\%$. A group reads a $10\ \text{mL}$ cylinder from a standing position and records $5.3\ \text{mL}$ while a partner at eye level reads $5.1\ \text{mL}$; the $0.2\ \text{mL}$ parallax bias is $4\%$ of the volume and shifts a $13.5\ \text{g}$ sample's density from $2.65$ to $2.55\ \text{g/cm}^3$.
Key Insight
Bend down so your eye is level with the water, because reading from above makes the volume look bigger than it is. The meniscus is the same phenomenon that pulls water up a paper towel, adhesion winning at the edges. Every systematic error has a direction: reading the meniscus from above always inflates volume and deflates density, so an experienced eye can name the mistake from the sign of the result alone.