An otaku (おたく) (plural usually otaku, though otakus is not unknown) is a variety of geek specializing in anime and manga.
The word is borrowed into English from Japanese and is derived from the Japanese o (お)(honorific) + taku (宅)(house, home). Because it is modern term, it is usually written in hiragana. It was used in the 1980s as a second person singular pronoun among hobbyist photographers. As it gained familiarity, the word was soon used by others to refer to hobbyist photographers. Since the photographers were seen as socially unskillful, reclusive, and obsessed with their hobby, otaku picked up those negative connotations and eventually was used to refer to any reclusive, obsessive hobbyist. A common stereotype is the young male otaku who lives at home without a job, has few social contacts outside of his otaku friends, and, who may be on the verge of becoming a dangerous stalker. This was further reinforced by the case of Tsutomu Miyazaki, a serial killer who raped four small children and ate parts of their bodies. He was revealed to be an otaku after his arrest in 1989.
Besides anime otaku (who sometimes enjoy many days of excessive anime watching with no rest) and manga otaku, Japanese culture has many other varieties, such as pasocon otaku (personal computer geeks), geimu otaku (playing video games) and otaku that are extreme fans of idols, heavily promoted singing girls. Otaku has much stronger negative connotations in Japanese than it does in English, and some American anime fans have adopted the term and use it proudly.
Otaku culture outside Japan often makes extensive use of Japanese loanwords. This can create an effect that is similar to Engrish, where the otaku will use Japanese phrases in conjunction with English. This usage is sometimes dubbed "Japanglish".
Anime and manga are two English words that were once solely otaku vocabulary, but have become common English. Before the term "anime" was adopted into English, they were simply called cartoons (e.g. Speed Racer).
wai - Often used as "wai-wai", sound of excitement
Also "glomp" (to latch onto or hug with much enthusiasm) is a word that is often used in anime fandom. (The origin of the word is said to come from the anime series Tenchi Muyo, where the character Ryoko would at times overzealously hug main character Tenchi, and the onomatopoeia of said hug would be 'glomp'.)