Pure Water（聖水 Seisui）is the name of a (temporary) stat-boosting item in the Fire Emblem series of games. The Pure Water items is one of the original combat-use items of the series, in the beginning of the series very few items existed that were useful in combat other then the Vulnerary. However the Pure Water offers a decent effect that can come in handy in several situations.
The Pure Water raises a character's resistance and gradually drops down to normal. Right after use a number will appear next to the character's resistance stat with an addition sign (+) in between them. In most games the bonus will begin with a +7 and drop by 1 each turn after the Pure Water has been used. In all the games Pure Water is in (with the exception of Fire Emblem: Thracia 776) Pure Water has three uses.
In the original games, Pure Water saw less as a character's inventory was limiting and they needed the room for all important weapons. However in the later games in the series such as Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn characters have much larger inventories leaving room for any extra items that may come in handy, such as the Pure Water.
Pure Water is usually given to characters with low resistance (for the obvious reason). Player usually like to give it to Knight/Generals since resistance is their major weakness, when given the resistance boost they become a very effective fighter unit.
In Fire Emblem: Thracia 776 it has 1 use and permanently raised the user's Magic stat.