What is the name for a company formed only to make an investment?

Perhaps this might be what you're after. For the USA, from Wikipedia:

Investment clubs are generally formed as general partnerships, but could also be formed as limited liability companies, limited liability partnerships, corporations, or sole proprietorship that transfer real estate assets to a group living trust (similar to a family trust). While an investment club could incorporate, the double tax treatment on corporate distributions makes the corporate structure less desirable than a partnership except in the case when a C Corporation pays out qualified dividends after deducting allowable expenses. Typically, a general partnership does not generate any tax liability on its own; instead, any tax liability is passed through to members each year. However, income taxes are generally much higher than taxes on qualified dividends.

In order to understand the legal structure that an investment club should choose, the club should first understand its club type. Each of the different club types will have different legal requirements as well as different reporting requirements. Typically, the SEC only requires reporting for investment groups with over 100 members, which is reclassified as an investment group, not an investment club. Publicly held offerings like a Real Estate Investment Trust as known as a REIT also have additional reporting requirements.