The participants you use in your study or experiment are your subjects."Within subjects" usually occurs in "within subjects design".
"Within subjects design",also called "repeated measures design",is a kind of experimental design in which the same group of subjects is exposed to all the different treatments in an experiment.
For instance,you want to determine whether the color of a drink affects people's perception of its sweetness.If you use a "within subjects design",you can give your participants two kinds of orange juice - a light orange color (Treatment 1) and a darker shade of orange (Treatment 2).The two drinks are identical except for the color.You can then ask all of your participants to taste each beverage and rate its sweetness.
One of the advantages of this type of experimental design is that if your participants say that the darker orange juice is sweeter than the light colored juice,you will know that the result is due to the difference in color of the two drinks rather than individual preferences.Another advantage is that this type of design requires a smaller number of participants.All you have to do is get a group of people and have them undergo all the treatments.If you use a different kind of design,say,"between subjects design",then you will need twice as many participants since you need two groups.