Overheal is when a player is past their regular max health. This will decay at a rate of 2 Health a second, until the user is back to their regular max health. Overheal a player as a Medic grants half the Ubercharge rate than healing a(n) injured player. Overheal has a max of 50% extra of the players max health, such as a normal Heavys health being 300. 300 divided by 2 (one half) is 150, so 150 + 300 is 450, making the max overheal for a Heavy 450. Some weapons can grant the user overheal without the use of a Medic, such as the Conniver's Kunai (max overheal is still 185, despite the lowered health) , and more. Some weapons grant health on hit/kill, such as the Blutsauger, or the Black Box, but do not give overheal. Also, when a person is overhealed, they will give off small cross particles that are dependant on the color of the team the player is on, the particles are red for RED or blue for BLU respectively. This shows up to both teams when a Spy is cloaked, whether disguised or not. On-hit or on-kill effects, such as the Blutsauger or Powerjack do not grant the health if the spy is using the Dead Ringer, on a disguised spy until he is dead, or a cloaked spy until he is also dead.
The Quick-Fix's overheal is only 25% more health, which is 50% less overheal than a medigun would give., so a heavy will have 375 health at full overheal from the quick-fix, as 300 divided by 4 is 75. The Quick-Fix can still maintain overheal from another source without it draining.
The Vaccinator has a 66% slower overheal rate, meaning that once the healing patient's health is over 100%, the healing from that point will be 66% slower. Interestingly, this does not affect the rate of ubercharge building.