Graduated Cylinder
A graduated cylinder is a tall marked tube for measuring liquid volume precisely, read at the bottom of the meniscus at eye level.
Definition
A graduated cylinder is a tall, narrow tube with lines on the side, called graduations, for measuring how much liquid you have in milliliters. It is calibrated glassware available from $10\ \text{mL}$ to $1000\ \text{mL}$ with graduations sized to match, read at the bottom of the meniscus with the eye level to the surface and one digit estimated beyond the smallest graduation, and the most precise reading comes from a cylinder only slightly larger than the volume needed. Cylinders are calibrated "to contain" at a stated temperature, usually $20^\circ\text{C}$, and a $100\ \text{mL}$ cylinder graduated every $1\ \text{mL}$ can be read to about $\pm 0.2\ \text{mL}$. Parallax from reading above or below eye level, residual liquid on the walls, and temperature-driven expansion are the main systematic errors, and volumetric flasks and pipettes replace cylinders when higher precision is needed.
Example
To measure $50\ \text{mL}$ of water for a plant, pour until the curved top of the water lines up with the $50$ mark, then set the cylinder on the table and bend down so your eye is level with the line. In a $100\ \text{mL}$ cylinder the meniscus sits between $23$ and $24$, closer to $23$, so record $23.4\ \text{mL}$; to find a rock's volume, note the starting level ($20.0\ \text{mL}$), lower the rock in on a string, and read $25.0\ \text{mL}$, a difference of $5.0\ \text{mL}$. That $5.0\ \text{mL}$ at $\pm 0.2\ \text{mL}$ carries a $4\%$ relative uncertainty versus $0.07\%$ from the balance, and a $10\ \text{mL}$ cylinder graduated every $0.1\ \text{mL}$ cuts the volume uncertainty to about $1\%$, reducing the density uncertainty fourfold with no change in the balance.
Key Insight
Tall and narrow beats short and wide for measuring, because the same amount of water moves the level much farther in a skinny cylinder. A beaker's markings are rough guides for approximate amounts and a cylinder's are for measurement, so never record a volume from a beaker in a data table. Match the glassware to the volume; measuring $5\ \text{mL}$ in a $250\ \text{mL}$ cylinder is like measuring a pencil with a tape measure marked in feet.