Otaku: a colloquial term for someone obsessively interested in a particular hobby or field; it can be used neutrally or pejoratively depending on context.
彼はアニメオタクで、週末は同人誌即売会によく行く。
He's an anime otaku and often goes to doujinshi sales events on weekends.
「オタク」は自分で名乗る場合と、他人が呼ぶ場合でニュアンスが違うことがある。
The nuance of 'otaku' can differ depending on whether someone calls themselves that or others call them that.
最近はオタク文化が一般に受け入れられ、ポジティブに使われることも多い。
Recently otaku culture has become more accepted and the term is often used positively.
彼女は鉄道オタクだから、ダイヤの話になると目を輝かせる。
She's a train otaku, so her eyes light up when the timetable is mentioned.