A ten-day period (approximately one third of a month). Used as in 'early/mid/late month' (reading: jūn).
来月の上旬（上旬＝ひとつの旬）に出張があります。
I have a business trip in the early part of next month (early = one ten-day period).
旧暦では一旬は十日間と定義されている。
In the old calendar a 'jūn' is defined as ten days.
A ten-year period (an old way of counting years, e.g., 'one jūn' = ten years). This usage is uncommon today.
古い文献には一旬を十年として数える記述がある。
Some old texts count one 'jūn' as ten years.
彼は四旬（よんじゅん）＝四十歳で引退したという記録が残る。
There is a record that he retired at 'four jūn' (i.e., forty years old).