noun, noun which may take the genitive case particle 'no', adverb
old days; the past; a long time ago
Refers to a time long before now, often nostalgically or in contrast with the present. Can be used as a noun, adverb, or with の to modify nouns.
昔、ここには大きな森があった。
Long ago, there was a big forest here.
昔の写真を
Looking at old photos makes me feel nostalgic.
彼は昔と変わらず元気だ。
He is as energetic as ever, just like in the old days.
noun
the past ten years; the past decade
Used as a unit of time, often in expressions like ひと昔 (about ten years) or ふた昔 (about twenty years). Not used alone in this sense.
See also: ふた昔
ひと昔前は、スマートフォンはまだ珍しかった。
A decade ago, smartphones were still rare.
ふた昔も前の話だ。
That's a story from twenty years ago.