1The year before last — i.e. two years ago, counted from the current year. Functions as both a noun and a time adverb. The reading おととし is the everyday/colloquial one, while いっさくねん has a more formal, written feel. Forms a set with 一昨日 (the day before yesterday) and 明後日 (the day after tomorrow).
一昨年、大学を卒業しました。
I graduated from university the year before last.
おととしの夏は本当に暑かったね。
The summer the year before last was really hot, wasn't it.
この店は一昨年オープンしたばかりだ。
This shop only opened the year before last.
おととし買った車がもう故障してしまった。
The car I bought the year before last has already broken down.
本社は一昨年の春に東京から大阪へ移転した。
Our head office relocated from Tokyo to Osaka in the spring of the year before last.