A fair test in science is an experiment conducted in a manner so that it does not provide any advantages to any of the conditions or subjects being tested. A fair test in science can also refer to a science test or exam that is administered fairly. A fairly administered test should be accessible to all test takers regardless of their disabilities or other personal characteristics.
A fair science experiment compares outcomes from a number of trials. For instance, if one wants to prove that sunlight makes people sneeze, they need to test a significant number of people to ensure that they have enough data to determine a causal relationship and not just a coincidence.
A fair experiment typically has one variable and several constants. For instance, if one were trying to determine the perfect amount of yeast to put in a loaf of bread, the variable is the yeast. All of the other ingredients need to stay constant.
If a teacher is trying to administer a fair science test or exam, they need to ensure that the students taking the test are not inherently disadvantaged by its format. For instance, if there are deaf people in the room, an orally-administered science test is not fair.