Concept:
A placebo is commonly used in clinical studies and sometimes produces psychological benefit.
Step 1: Placebo is an inactive substance.
Step 2: It looks like a real medicine but does not contain active drug.
Step 3: It has no specific pharmacological action.
Step 4: It may produce benefit due to the patient's belief or psychological response.
Step 5: Therefore, placebo is an inert substance with no specific pharmacological action.
\[
\boxed{\text{Inert substance with no specific pharmacological action}}
\]