The one factor that can be manipulated by the experimenter

It appears that you are referring to a variable in this question. There are two types of variables, dependent (DV) and independent (IV). The IV would be the thing the experimenter is testing - of the cause of any changes throughout the course of the experiment. The DV is the result or effect of the IV and it is usually something that we can measure. The IV is the only thing that we can change or control during an experiment whereas the DV will change in response to your IV (but not something the experimenter is changing directly). We can use other names to describe DV and IV. Sometimes the IV is referred to as the manipulated variable and the DV is called the responding variable. 

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