Overheal

From TF2 Wiki

An overheal refers to the state in which a player's health is at 150% of their maximum. Medics can overheal players with their Medigun. The player's health cross on the HUD will glow more brightly as he or she is more overhealed.

A full overheal will fade away in exactly 20 seconds for all classes, provided that the player does not take additional damage. While a Heavy's health can be boosted to 450, it will return to its base of 300 at the same rate as a Scout, whose health can be boosted to 175 from 125.

If a Medic heals a character already at 100% health, it will take 1 to 3 seconds to raise them to 150% health (overhealed). The time it takes depends on the class; those with the highest starting health takes the longest to overheal.

During setup time, or while approaching an objective, a Medic can keep multiple teammates on the front line overhealed instead of only their primary healing target. This also increases the ÜberCharge bar.