The day before yesterday (two days before today). Commonly read as 'おととい'; the formal reading 'いっさくじつ' is used in written language.
一昨日、久しぶりに友達に会った。
I met a friend I hadn't seen in a long time the day before yesterday.
一昨日は雨が降っていたから、散歩に行かなかった。
It was raining the day before yesterday, so I didn't go for a walk.
一昨日買った本をまだ読み終えていない。
I haven't finished the book I bought the day before yesterday.