1. A classic cat girl pose thats originated in Japan quickly becoming world wide popular though anime/manga culture. The pose is simple, pully your arms close into your bosy with your elbows pointing downward. Then using one or two hands, make a fist and put them up to your neck/cheeks with your palms facing outwards.
Sometimes cosplayers/otaku will also wear a cat ear headpiece or accessory accompanied with a choker/collar with a bell on it. The word "nyan nyan" is commonly used in japan to decribe the noise a cat makes. To seal the deal, often they will say the phrase "nyan nyan" while taking the picture and smiling.
2. A sound a used to describe the noise a cat makes. Commonly used in Japan.
Yesterday at the anime convention we saw many girls dressed up and posing like cats." "You mean a nyan nyan pose like this?