在 - TO EXIST on the Earth

Readings & Meanings:

あ（−る）

ザイ

to exist, country, suburbs

Common Usage:

現在

存在

Examples:

タイムマシンがあれば、過去、現在、未来を自由に行き来できる。

彼はクラスの中で存在がある。

Translations:

If there were time machines, we could freely come and go into the past, the present, or the future.

He is present in class. (Whether he speaks or not, everybody is aware that he is there. He has a presence.)

Important:

If you want to use 在る in place of the normal, hiragana ある, be careful, you will sound like an academic. Also, be careful of the other あるs:

在る - physical existence...

有る - more often used, a general existence

或る - this is different from 有る and 在る, and is not the verb, just an adjective meaning "a certain, one". Example: 或る女 - A Certain Woman

References:

http://www.thejapanesepage.com/kanji/kanji/i.htm