What is the definition of an independent and dependent variable for science?

Answer 1

An Independent variable, or IV, is the thing you change. The dependent variable, or DV is the thing that changes because of the IV.
The Independent Variable is the Variable that is purposely changed, it is the one manipulated. and the Dependent Variable is the Variable that reacts to the changes due to the independent Variable.

For Example:

in a study where you will find out which age of Compost will be more effective on Tomato plants. you have Compost A (5 months old Compost), Compost B (1 year old Compost), and Compost C (1 and a half year old Compost). you will apply these compost on Flat A, and Flat B (both Flats containing the same amount of Tomatoes and same type of soil). Flat C will be your control variable.

Your Independent Variable is the Age of Compost

And Your Dependent Variable is the Growth (Color, Size, Weight) of the Tomato plants.

so basically your independent variable causes change in the dependent variable.