What is the definition of an independent variable?

I just don't understand what it means? Everyone keeps telling me that it's the variable that changes but what does THAT mean? Can you please help me understand the concept in a clear way? I have to take a test tomorrow and I need help!What is the definition of an independent variable?
independent variable is the variable that the RESEARCHER changes on purpose to see if it makes a difference on the outcome.

For example, lets say you, the researcher, want to know how many daily calories can you eat and still lose 2 lbs a week. So, you would need to try several different calories levels. You test eating 1200 calories for one week, 1600 calories for another, and 2000 calories for another week. You independent variable would be: amount of calories (because this is what you changed on purpose to see if it makes a difference on the outcome - the outcome in this case would be number of pounds you lost each week)What is the definition of an independent variable?
Basically the independent variable really is the thing that you change. For example, if you decided to make an experiment where you wanted to see if boys were better at Maths than girls. (Not saying they are, but just an example.) You could give both of them a test in Maths and record their marks. The independent variable is basically the gender. Because that's the only thing you've changed to actually have a point in doing the experiment. Hope this helped :)

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