An onomatopoeic word for 'meow', a cat sound. The more common form is 'nya', a less common form is 'nyu'.
'Nyo' (にょ) is also acceptable and has been popularised by Dejiko-chan (a catgirl, or nekomimi (猫耳) in the anime TV series DiGi Charat. Dejiko-chan ends almost all sentences with this suffix (see examples). Another feline character in the show, Puchiko-chan, ends most of her sentences with the suffix 'nyu'.
Is is a misconception that 'nyo' can be defined as 'urine'. The Japanese word for urine is 尿 (nyō), the vowel sound being slightly longer. This is a kanji character whereas onomatopoeic words in Japanese are usually spelled only in hiragana.
It would be akin to using the English sentence "We are buying a cat" but pretending one had said "Urine are buying a cat", simply because the words use similar pronunciation.
"Ah this voice! It's that laughing ghost-nyo!"
"No-nyo! Too much trouble-nyo!"