Control Variable
A control variable is any factor held constant during an experiment so it cannot influence the result being measured.
Definition
A control variable is something you keep the same in every part of your experiment so the test stays fair. It is any factor that could affect the dependent variable but is held fixed across all conditions, which ensures that a change in the outcome can be attributed to the independent variable alone. Identifying control variables means thinking through every plausible influence on the result; those that cannot be held constant should be measured and reported, or randomized across conditions so they add noise rather than bias. A control variable is not the same as a control group: the control group is a comparison setup, while control variables are held equal in every setup, including the control group.
Example
Testing whether fertilizer helps bean plants, the control variables are water amount, soil type, pot size, and sunlight, and every plant gets the same of each. In a salt-dissolving experiment with temperature as the independent variable, the controls are salt mass ($5.0\ \text{g}$), water volume ($100\ \text{mL}$), grain size, stirring rate, and cup shape. In a ramp-angle experiment a group fixes the cart, the release trigger, the $0.50\ \text{m}$ release mark, the board, and the floor, and logs air temperature each session because they cannot hold it constant.
Key Insight
There is one independent variable but many control variables, and listing them before you start is how you catch unfair differences. An uncontrolled variable that happens to correlate with your independent variable is a confound, and it can produce a convincing but false result. Most surprising findings that fail to replicate trace back to a variable nobody thought to control.