From Bulbapedia, the community-driven Pokémon encyclopedia.
Valerie's Fairy-type Pokémon in the anime
A type expert (Japanese: タイプエキスパート type expert) is a Pokémon Trainer who specializes in just one type. These Trainers are most commonly Gym Leaders, Trial Captains, Island Kahunas, or members of the Elite Four. There are also some Trainer classes that specialize in one type, such as the Bug Catchers who specialize solely in training and battling with Bug-type Pokémon.
In the Pokémon anime, two former traveling companions of Ash aim to become type experts, with Misty wanting to specialize in the Water type and Iris aspiring to be a great Dragon-type specialist. In Pokémon Black 2 and White 2, the Type Expert Tournament at the Pokémon World Tournament features type experts from all of the core series regions known at the time.