wan わん

In Japanese, wan わん is the sound dogs make when they bark, an onomatopoeia. In other words: wan is "woof" in Japanese, or wanwan わんわん means "bow-wow" in Japanese, or something like that.

Note that wan わん doesn't have kanji and may be written with hiragana because hiragana is cuter, or written with katakana as wan ワン. In the case of wan ワン, beware that it's homonym with the katakanization of the English number "one." That is, wan piisu ワンピース means "one piece," and probably not "woof piece."

When wan' わんっ or wan' ワンッ are written instead, with a small tsu っ at the end of the phrase, it emphasizes the sound, expressing a stronger-sounding bark.

By the way, dogs make other noises too, not just wan, and so the following words also exist:

• gurururu グルルル
  Grr.
• uofu ウォフ
  uufu ウゥフ
  Woof. Roof.
• kyaun キャウン
  kyuun キューン
  Nnn.. nnn.. nnn... (crying sound.)

The word nyan にゃん would be the sound cats make in Japanese, "meow." And the word wanko わんこ, derived from wan, would be a cutesy way to refer to a "doggy."