What is a independent variable?

Answer 1

in a experiment, the factor that is deliberately manipulated
An independent variable is the variable that changes and the dependant variables are the variables that don't change. So that would mean that the independent variable is typically the variable being manipulated or changed and the dependent variable is the observed result of the independent variable being manipulated.
An independent variable is a variable in an experiment that is changed to test a hypothesis. The dependent variable relies on the change of the independent variable to change itself.

Answer 2

As an example, consider an experiment to find out how the amount of sugar that can dissolve in a cup of water depends on the temperature:

Bring a cup of water to 30C, and gradually add sugar while stirring, until a little is left that won't dissolve. Repeat for 40C, 50C, etc. The amount of sugar dissolved is called the dependent variable, since it depends on the temperature of the water. The temperature is called the independent variable because it doesn't depend on anything ... you decide what it will be.

By convention, the independent variable - which you control - is plotted on the horizontal, or x-axis. The dependent variable - which you measure - is plotted on the vertical, or y-axis.