In scientific experimentation, a fixed variable is a variable that remains constant throughout the experiment. A fixed variable is more commonly known as a control variable.
During scientific experimentation, it is extremely important to perform each part of the experiment in an identical manner to the last in order to prove that the results were directly influenced by the single variable that changed. A fixed variable is anything experimenters can ensure stays the same throughout experimentation, such as ambient room temperature or brand of chemical. Any variable that is immeasurable or uncontrollable is considered an extraneous variable, and ideally has little effect on the results of the experiment.