A statistic, or stat for short, is an element which determines certain aspects of battles in the games. Stats may also refer to the numerical values of each field in regards to individual Pokémon.
Each Pokémon has the following stats, in- and outside of battle:
The Hit Points, or HP for short, determine how much damage a Pokémon can receive before fainting. It is the most visible of the stats in battle, appearing both graphically (as a bar with a fill colored green, yellow, or red depending on how much HP in percentage is left) and as a current/max amount below the bar.
If a Pokémon has more than half of its HP left, its HP bar remains green. If the Pokémon has between one-fifth and half of its HP, the bar will turn yellow. If a Pokémon has less than one-fifth of its HP remaining, the bar will turn red and a series of 4 beeping sounds will notify you that your Pokémon is in danger of fainting.
The Attack stat partly determines how much damage a Pokémon deals when using a physical move.
The Defense stat partly determines how much damage a Pokémon receives when it is hit with a physical move.
The Special Attack stat or Sp. Atk for short, partly determines how much damage a Pokémon deals when using a special move.
The Special Defense stat or Sp. Def for short, partly determines how much damage a Pokémon receives when it is hit with a special move.
The Speed stat determines how quickly a Pokémon can act in battle. Pokémon with higher Speed will make a move before ones with lower Speed under normal conditions; in the case that two Pokémon have the same Speed, one of them will randomly go first.
In-battle stats are only effective in battles. They are reset once the battle is finished.
The evasion rate, or evasiveness, of a Pokémon's move determines its probability of missing the target. The initial value at the start of any battle is 100%. If the evasiveness is decreased below 100% with a move such as Sweet Scent, then the opposing Pokémon has a better chance of its move hitting the opponent. If the evasiveness is increased above 100% with a move such as Double Team, the opposing Pokémon will have a harder time connecting its moves.
The accuracy rate of a Pokémon's move determines its probability of hitting the opponent. The initial value at the start of any battle is 100%. The accuracy rate can be raised with accuracy-raising moves, such as Hone Claws.
A Pokémon's base stat values will most often have the greatest influence over their specific stats at any level. Disregarding individual values, effort values, and Nature, a level 100 Pokémon's stats in Attack, Defense, Special Attack, Special Defense, and Speed will be exactly 5 more than double its base stat values in each, while the HP stat will be 110 plus double the base stat value (except in the case of Shedinja, which always has a HP of 1).
When a Pokémon grows a level, its stats will increase. For each level gained (ignoring Nature), stats will increase by 1/50 the base stat value, and 1/100 the combined individual value and effort value. This means that it is impossible, through leveling up, for a Pokémon to ever lose points in a stat unless it evolves into a Pokémon with a lower base stat value for that specific stat or an EV-reducing Berry is used. It is also impossible for any Pokémon other than Shedinja not to gain HP upon leveling up without using a Pomeg Berry, as no evolutionary line has members with a lower base HP than the pre-evolved forms.
Most Natures enhance the growth of one stat, while hindering the growth of another. After all other calculations are finished, the stat that the Nature enhances will be 110% of what it would be without the Nature, and the stat hindered will be 90% of its normal value.
Individual values, or IVs for short, cause two Pokémon of the same species to have different stats.
Effort values, or EVs for short, are what cause a trained Pokémon to have higher stats than an untrained counterpart of the same level. For every 4 EVs gained, a level 100 Pokémon will have 1 extra point in its stats. Variance of stats caused by EVs reaches a maximum of 63 points at level 100, as a Pokémon can gain a maximum of 252 EVs in a single stat.
When a stat is used in a calculation in battle, a number of modifiers may be applied during the calculation. During a battle, a Pokémon's effective stats may be raised or lowered by certain moves, Abilities, and held items. Some attacks may only have a chance of raising or lowering stats, while certain Abilities and held items may require a triggering event to activate any stat modifications.
The modifiers conferred by moves operate on a sliding scale of stages. When a given stat is raised or lowered, its current stage is increased or decreased by the amount dictated by the move, up to a maximum of +6 or a minimum of -6. A given stage corresponds to a given multiplier that will modify the stat when it is used in battle calculations.
While some Abilities and held items may also modify the stages of a Pokémon's stats, others may apply unique multipliers that are separate from—and thus can stack with—stages. Examples include the Ability Slow Start, which temporarily halves Attack and Speed, and the held item Choice Band, which boosts Attack by 50%.
A few items may also modify a Pokémon's stats outside of battle by increasing or decreasing the EVs for that stat. Vitamins increase EVs, while EV reducing Berries (like the Pomeg and Tamato Berry) reduce EVs.