Definition (Cambridge Dictionary):

1. spoken words that are thought to have magical power, or (the condition of being under) the influence or control of such words 【咒语，符咒；魔法；着魔】

2. a period of time for which an activity or condition lasts continuously【(持续的)一段时间】

3. a short period of a particular type of weather【(特定天气的)一段短暂持续时间】

Example:

The witch cast/put a spell on the prince and he turned into a frog.

女巫对王子施了魔法，把他变成了青蛙。

I lived in Shanghai for a spell.

我在上海住过一阵子。

There has been a long spell of dry weather.

干旱天气已持续很长一段时间了。