An obsessive fan — of anime, trains, idols, anything.
In Japanese it needs a subject: アニメオタク, 鉄道オタク, アイドルオタク. It still carries more edge at home than the English "otaku" does abroad — though younger speakers increasingly use it proudly, and the clipped form ヲタ shows up online.
From お宅, a formal word for "your house". 1980s fans addressed each other that distantly, and the word stuck to them.
Can still sting in Japan, unlike in English where fans wear it happily. Safe about yourself, riskier aimed at someone else.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to Japanese — it uses オタク the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook Japanese.
Free to start. No signup for your first conversation.