Keiretsu
A Japanese term describing a loose conglomeration of firms sharing one or more common denominators. The companies don't necessarily need to own equity in each other.
Investopedia Commentary
This term has been in the news every now and then, especially when they talk about Silicon Valley. One example would be the close relationship between AOL and Sun Micro. The two firms don't have ownership in each other, but they work closely on various projects.
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See also: Joint Venture, Partnership, Silicon Valley, Strategic Alliance
Also spelled: keiritsu