Basically, a variable is any factor that can be controlled, changed, or measured in an experiment. It's anything that can be changed in the experiment.
Explanation:
Scientific experiments have several types of variables. The independent and dependent variables are the ones usually plotted on a chart or graph, but there are other types of variables you may encounter. The presence/absence of the chemical is the independent variable. The health of the (ex:)rat (whether it lives and can reproduce) is the dependent variable.
