Kansai is not a prefecture, it is a region encompassing a number of prefectures. The literal meaning of the word is "west of the barrier", with barrier meaning the (imaginary) wall dividing the west (Kansai) from the east (Kanto) of Japan; actually it's more a division between the Kyoto-Osaka region where the focus of power and history in Japan used to be, and the Edo/Tokyo region that was always economically powerful but used to be considered wild and uncivilized until the Tokugawa shogunate reversed the power balance.

Nowadays, Tokyo is the center of Japan, and Kansai people are seen as somewhat odd, and funny, partially because of their distinctive accent, Kansai-ben.

関西

Kansai [Jp. kansai, "west of the barrier," cf. Kanto.]

1.

The megalopolis formed by the cities of Osaka, Kyoto, and Kobe, Japan.

2.

The western portion of Honshu Island, generally including the Yamato plain but nothing farther east.

3.

The region of Japan in which the Kansai dialect is spoken.