Medical leave was a term in Starfleet which described shore leave at medical order. According to Starfleet regulations, an officer taking medical leave could select their rehabilitation facility.
Although shore leave can be ordered by medical personnel for a crewmember's well-being, a medical leave may be specifically for those who suffered physical or psychological injury in the line of duty.
In 2267, Dr. Leonard McCoy good-naturedly accused Fisher of falling and injuring himself on purpose so he could get a vacation. (TOS: "The Enemy Within")
Later on, Captain James T. Kirk visited the Shore Leave Planet after being informed by Spock that his reaction time and associational reading were poor. (TOS: "Shore Leave")
Also that same year, Montgomery Scott took medical leave on Argelius II after sustaining a head injury. (TOS: "Wolf in the Fold")
In 2375 Ensign Nog was ordered to take medical leave by Starfleet Command after he lost a leg in the battle of AR-558. Upon his return to Deep Space 9 Captain Benjamin Sisko read the orders from a PADD and told Nog that his medical leave was granted with the exception of his participation in his welcome home party in the wardroom. (DS9: "It's Only a Paper Moon")