回 - As the Wheel TURNS

Readings & Meanings:

エ

カイ

まわ（る）

まわ（す）

a turn, to turn

Common Usage:

今回

一回

回復

Examples:

今回だけは見逃してやろう。

人生に一回あるかないかの絶好のチャンスだ。

彼は若いだけあって、回復が早い。

Translations:

I'll let it go this time.

This might be a once-in-a-lifetime opportunity.

Since he's young, he's recovering quickly.

Radical:

I don't know if this one qualifies as a radical. Sure, you could say it's got くち（口）. Also, like 国, it's くにがまえ.

But let's just appreciate the pictograph, here. This is clearly something that turns.

I acknowledge that it's not round, but kanji doesn't have circles in it, unless you count calligraphy art.

References:

http://www.kanjinetworks.com/indices/radindex.html
http://www.thejapanesepage.com/node/kanji/e.htm