A Physical is a medical procedure done aboard starship to analyze a crew member's health.
Physicals can be ordered by the Chief medical officer when ever he or she feels that a crew member may need it, including the Captain. Aside from ordered physicals starships also have regular physicals to monitor the regular health of the crew. In the 23rd century these physicals were done quarterly, by the 24th century they were reduced to Annual physicals. (TOS: "The Corbomite Maneuver"; TNG: "Allegiance")
In 2267, aboard the Enterprise, Doctor McCoy had Nurse Chapel conduct a complete physical on Ensign Chekov. (TOS: "The Deadly Years")
See Also: "I, Mudd", "Trials and Tribble-ations"