What is meant by an independent variable?

Independent variable is a concept that helps us understand the cause-effect relationship in the experimental design. For example, if we interest in knowing if omega three oil could improve attention. Under this research statement, the cause is omega three oil, and the effect is attention improvement. In the experimental design, researchers will recruit a pool of participants. Researchers will give half of the participants omega three oil tablets, and the other half will receive dummy tablets (without omega three oil). In this case, the presence and absence of omega three oil is the independent variable. By comparing the attention ability of these two groups of participants, researchers could find out if omega three oil could improve attention.