A variable that changes depending on another variable.
Example 1: What is the dependent variable in the following scenario?
Dan is placing ice in a icebox while measuring its temperature. For every 5 blocks of ice that Dan places in the icebox, the temperature increases by 2 degrees Fahrenheit.
Because the temperature increases when the ice is added, temperature is the dependent variable. Note that it would not make sense to say that ice is added because the temperature increases.