If something is periodic, it happens again and again, usually with a fairly regular gap in between — a periodic check, periodic meetings, periodic rain in the afternoon. It does not have to be perfectly clock-like; the point is that it returns, rather than happening once.

If something is periodic如果一件事是periodic的，it happens again and again它會一再發生，usually with a fairly regular gap in between中間通常間隔得相當規律 — a periodic check, periodic meetings, periodic rain in the afternoon像periodic check定期檢查、periodic meetings定期會議、periodic rain in the afternoon下午常態性的陣雨。It does not have to be perfectly clock-like它不需要像時鐘一樣分毫不差；the point is that it returns重點是它會一再出現，rather than happening once而不是只發生一次。

It is a slightly formal, textbook-ish word. In everyday talk people more often say “regular,” “every so often,” or “from time to time.” Scientists also use it for things that follow a repeating pattern (that is why the periodic table has that name: the properties come round again in a cycle).

It is a slightly formal, textbook-ish word這是個稍微正式、有點教科書味的詞。In everyday talk people more often say "regular," "every so often," or "from time to time."日常講話大家比較常說regular、every so often，或from time to time。Scientists also use it for things that follow a repeating pattern科學家也會用它來說會反覆出現規律的東西 (that is why the periodic table has that name: the properties come round again in a cycle)（這也是periodic table元素週期表名字的由來：性質會週期性地重複出現）。

Though actually, there is a different, rarer chemistry word periodic (as in periodic acid), built another way, from per- + iodic; that is not this meaning.

Though actually不過其實，there is a different, rarer chemistry word periodic還有一個不同的、比較少見的化學用詞periodic (as in periodic acid)（像periodic acid過碘酸），built another way是用不同方式構成的，from per- + iodic由per-加上iodic組成；that is not this meaning那不是這裡的意思。