Wild

WILD
From Bulbapedia, the community-driven Pokémon encyclopedia.

Wild Pokémon (Japanese: 野生ポケモン wild Pokémon) are any Pokémon that are not currently owned by a Pokémon Trainer. They are encountered by walking in tall grass or caves, traveling with Surf, headbutting trees (Generation II only), breaking open rocks with Rock Smash (in Generations II and III), by fishing, or by slathering Honey on Honey Trees and checking back later (Generation IV only). Wild Pokémon can also appear when using Sweet Scent or Honey whilst standing stationary in grass.

Wild Pokémon can be caught using any variety of Poké Balls.

Wild Pokémon often do not know the best moveset for their level, but instead know the moves that were the most recently obtainable. Some wild Pokémon who can learn almost any move by way of an ability or special move (such as Smeargle) won't use this ability before being captured (i.e. Smeargle cannot use Sketch until the player battles with it, making that the only move it will know in the wild).

Also, in Generation II, listening to Pokémon March on the radio increases the probability of encountering wild Pokémon when walking through the grass, in caves or surfing, and listening to Pokémon Lullaby will decrease the probability. In Generations III and IV, several Pokémon abilities have the same effects when those Pokémon are in the lead spot.

In the first anime episode, Ash's Pokédex states that wild Pokémon tend to be jealous of human trained Pokémon. Also, In Diamond and Pearl, Pokémon can tell (through footprints) Dr. Footstep that wild Pokémon jeer that trained Pokémon have forgotten the wild.

Generation IV
In Pokémon Diamond and Pearl, wild double battles were introduced. There are five instances that this can happen.

* Eterna Forest (with Cheryl) * Wayward Cave (with Mira) * Iron Island (with Riley) * Victory Road (with Marley) * Stark Mountain (with Buck)

The protagonist will encounter another character that needs help getting through the said location. When the Trainer encounters a Pokémon with this person, it will be a wild double battle.

When this wild double battle occurs, the player cannot capture a Pokémon until one of them has fainted. After the protagonist goes through the area the first time, the wild Pokémon will only appear in single battles.