Final answer:
A placebo is an inert substance used to mimic the treatment in a control group within an experiment, helping to distinguish the true effects of the treatment from psychological effects.
Step-by-step explanation:
An inert substance that resembles the treatment given to an experimental group in an experiment is known as a placebo. This is often used in a control group to help distinguish the actual effect of the treatment from the psychological effect of simply receiving a treatment (placebo effect). In a well-designed experiment, the control group receives the placebo, while the experimental group receives the actual treatment. This setup aids in reducing potential biases, such as expectancy, and helps ensure that any differences observed between the two groups can be attributed to the treatment itself, thereby strengthening the validity of the research findings.