An interval fund is a professionally managed, pooled investment vehicles that combine attractive features of both closed-end funds and traditional open-end funds. Griffin Capital provides,a sponsor of two interval funds,  provides the following table comparing interval funds to other common types of investments:

Interval funds typically operate as a fund of funds, offering investors exposure to a diversified portfolio of alternative investments through one 1940 act vehicle. Many major alternative investment managers, REIT and BDC sponsors have registered interval funds in recent months. For info, see our database of new fund launches.

A variety of regulatory and market pressures are driving the growth of interval funds. However, lack of data makes it difficult for investors to evaluate choices. Therefore, the purpose of Interval Fund Tracker is to increase transparency by providing comprehensive information to market participants. Furthermore, this site provides competitive intelligence and research services to asset managers.

What is an interval fund?

Officially, an interval fund is:

• A continuously  offered closed end fund.
• Required to repurchase between 5% and 25% of shares at NAV at predetermined intervals(usually quarterly).

More details are provided by the SEC website.
See also: Research Resources