What Is An Extraneous Variable And Why Does It Matter? byforbes.com

An extraneous variable is a variable that is not being tested. It can directly affect the dependent variable of your study. For example, if you were testing how much money you make per hour based on your age, the age of your children would be an extraneous variable. You are not testing whether or not your children affect how much money you make per hour, so in this case, they would be considered extraneous variables to the experiment.