Prefix used in compounds to refer to the previous or most recent instance of a time period, such as 昨年 (last year) or 昨日 (yesterday). Not used as a standalone word.
Last year's sales were the highest ever.
The situation last year was unstable.
作 (さく) means a work, harvest, or craftsmanship, while 昨 (さく) means 'previous' as in 昨年 or 昨夜; they share the same sound but are unrelated in meaning.
The kanji 昨 is composed of 日 (sun/day) and 乍 (while/although), originally indicating 'the day that has passed'. The reading さく is the on'yomi (Sino-Japanese reading).
one previous; the one before last
last winter
yesterday evening; last night
last spring; the spring of last year
early yesterday morning
yesterday morning
last period; previous period
yesterday's paper; yesterday's issue
last week