Otaku (Heavy fans of Japanese cartoons) slang for a cat girl. Literally meaning "Cat-ear" in Japanese. Usually a girl in cosplay (costume) wearing homemade cat ears and tail of their favorate character from an anime series (Japanese cartoon).
Note: Can also be someone of a cat-humanoid race, as in a mithra from Final Fantasy XI.
A more literal meaning of Nekomimi in Japanese is Nekomusume (cat daughter or cat girl).