1Something that is only natural, reasonable, or obvious — not surprising or special in any way. Very common in speech and often written in kana.
毎日練習しているんだから、上手になるのは当たり前だ。
You practice every day, so of course you're getting better.
「ありがとう」を言うのは当たり前のことだよ。
Saying "thank you" is just common courtesy.
そんなに怒られて当たり前だよ、約束を三回も破ったんだから。
It's only natural you got scolded like that — you broke your promise three times.
親が子供を心配するのは当たり前だ。
It's natural for parents to worry about their children.
2Ordinary; commonplace; the norm — describing something that is nothing special, just the usual state of things.
この会社では残業が当たり前になっている。
At this company, working overtime has become the norm.
今では一人一台スマホを持っているのが当たり前だ。
Nowadays it's completely normal for everyone to have their own smartphone.
海外では、店員が客に座って接客するのが当たり前の国もある。
In some countries overseas, it's normal for shop staff to serve customers while seated.