Immunity from poison, obtained by consuming series of small, gradually increasing doses of that poison.
This word comes from name of the King of Pontus, Mithridates VI (132-63 BC), who used this technique to protect himself from assassination by poison. When defeated by the Romans at Pompey he attempted to commit suicide by poison, but was unable to do so due to his well mithridatized state, and had to have a mercenary stab him.
This is also more or less the way we make antivenin, although these days we poison horses, not kings. We inject horses (or other animals) with increasing doses of venom, and once their bodies are making substantial amounts of antibodies against the venom, we harvest their blood.
The practice is probably best known to the average noder through The Princess Bride, where the famous Sicilian scene ends (Spoiler alert) with the revelation that Wesley has spent years building up a immunity to iocane powder. (End spoiler).
Other forms of the word are mithridatic, and mithridatize or mithridatise.