Which organization defines CJK Unified Ideographs? CJK means Chinese, Japanese and Korean. Well, which organization defines the standard?
On the Frequently Asked Questions page for Chinese and Japanese on Unicode there is a question that asks:

Q: Who is responsible for future CJK characters?

The answer reads:

A: The development and extension of the CJK characters is being done by the Ideographic Rapporteur Group (IRG), which includes official representatives of China, Hong Kong (SAR), Macao (SAR), Singapore, Japan, South Korea, North Korea, Taiwan and Vietnam, plus a representative from the Unicode consortium. For more information, see the IRG home page.
The IRG is very carefully cataloging, reviewing, and assessing CJK characters for inclusion into the standard. The only real limitation on the number of CJK characters in the standard is the ability of this group to process them, because the characters are increasingly obscure (no person knows more than a fraction of the set already encoded).

Each region has their own official representatives who helps in maintaining standards in connection with IRG.

Your titled also asks about unified standards. The same FAQ above has a separate question:

Q: What is the process for proposing new CJK unified ideographs?

which is answered:

A: Newly proposed CJK unified ideographs are first submitted to the IRG through national bodies or liaison organizations, and are then assembled into a new &quot;IRG Working Set&quot; that goes through several rounds of detailed review and scrutiny before being approved for standardization as a new CJK unified ideographs extension. Individuals who wish to propose the encoding of new CJK unified ideographs are encouraged to work with their respective country's national body.

The answer is more or less the same: regional organizations work with IRG on these issues.