An onomatopoeia (sound word) used for imitating a cat's cry. The Japanese equivalent of "meow".
This tag includes variations such as "nya", "nyaa", etc. In Japanese, it can be written as: にゃん, にゃ, にゃー, にゃあ, ニャン, ニャ, ニャー, ニャア, and probably a few other variations.
Often accompanied by a paw pose or other cat-like mannerisms.