EV training

Effort values (abbreviated EVs and previously called Stat Exp) are attributes which give bonuses to a Pokémon's stats and improve differently depending which Pokémon they defeat. These bonuses, in the form of effort points, are gained in addition to bonuses gained by increasing level. A Pokémon which increases in level using a Rare Candy instead of battling does not gain any EVs, making it weaker than a Pokémon who increases in level normally.

Roughly speaking, defeating fast Pokémon increases Speed better than fighting slow Pokémon, defeating Pokémon with high hit points improves HP more than defeating Pokémon with low HP, and so on. For example, fighting 100 Machop will improve a Pokémon's attack stat more than fighting 100 Abra of the same level, whereas the Abra will improve the Special Attack stat more. In Generation I and Generation II effort points given are equal to the Pokémon's base stats. For a list of the effort points that Pokémon give away on their defeat in Generation III and Generation IV, see list of Pokémon by effort value yield.
Effort values only appear in the main series Pokémon games, and are not present in the spin-off games, such as the Mystery Dungeon series.

min-maxing in Pokémon. Involves killing only specific species of wild pokémon that drop the desired stat boosts and running from pokémon that drop undesired ones until said stat gains the desired amount of Effort Values, or EVs, often with assistance of certain stat-gaining power items to boost only two or three stats instead of the intended six. Often frowned upon due to the casual nature of the game and how EV trainers would spend hours grinding and counting EVs on every one of their pokémon just to gain a slight advantage over others in competitive play.