Where is the CBD in Tokyo?

Need to know where the central business district is in Tokyo! Which 'suburb'/region is it in?

If I were to guess, it would be Marunouchi, Chiyoda-ku. But I don't really know what you mean when you say "Central Business District". Is this a term used in your country?

Marunouchi and Ohtemachi, Chiyoda-ku, would be for major financial institutions and rather traditional, long-standing corporations' head offices
Kasumigaseki, Chiyoda-ku, would be for national government agencies
Ginza, Chuo-ku, would be for commercial facilities (department stores and shops)
Shinjuku, Shinjuku-ku, is another commercial buildings and also Tokyo metropolitan government building
Harajuku and Shibuya, Shibuya-ku, would be for rather artistic institutions and shops and many small offices
... so different parts of Tokyo has "centers" with different inclinations; what are you looking for?

Tokyo 23-ku(special district ward) is the CBD in Tokyo.

Are you Australian? I've only heard fellow Aussies use this term. If you want to know what the 'central' part of Tokyo is in terms of things to do, eg: the most lively/bustling areas of Tokyo, then it's Shinjuku or Shibuya. The CBD is Marunouchi or Kasumigaseki, but these are the financial and political areas of the city which are boring unless you have a specific reason to go there.