In a scientific experiment, the variable manipulated or controlled by the experimenter is called the

The variable manipulated or controlled by the experimenter is called the independent variable.

Example:
If the flow velocity at the bottom of a tank is measured by varying the height of water in the tank, we are measuring velocity as a function of water height.
Therefore,
water height  =  independent variable (controlled)
velocity =  dependent variable (measured in response to water height).

Mathematically,
v = f(h)
where v =  response variable (dependent)
            h = controlled variable (independent).