investment company. A firm in which investors pool their funds to allow for diversification and professional management. Because individual firms often specialize in particular types of investments, the potential returns and risks vary considerably among firms.or example, a single investment company might offer an aggressive-growth fund, a growth and income fund, a US Treasury bond fund, and a money market fund. Or a closed-end investment company might offer an international fund focused on a single country, such as Ireland, or a region, such as Latin America.