N3
おきに
Repeated at intervals, Every other, Every, At intervals of
この薬は一日おきに飲んでください。
Please take this medicine every other day.
Explanation
「おきに」 follows a time, quantity or distance and means 'every other ...' or 'at intervals of'. A gap is left, then it repeats: 「1日おきに」 means skipping one day (the 1st, 3rd, 5th), so once every two days; 「1メートルおきに木を植える」 (plant trees one metre apart).
Mixed up with
Do not confuse it with 「ごとに」: 「1日おき」 skips a day; 「1日ごとに」 means every single day.
Strictly, 「1人おき」 and 「1日おき」 mean 'skip one person' and 'skip one day'.
Often tested in listening: a nurse says 「6時間おきに薬を飲んでください」. Remember it means 'with that gap between'.