"Unique" is a term bandied about on a regular basis on Wall Street, but in the case of so-called "interval funds", the shoe more than fits, description-wise.

Interval funds are a new kind of fund structure that periodically offers to buy back shares from shareholders, explains Cole Reifler, president and founder of Forefront Capital, an investment firm markets interval funds to investors for as little as $1,000. Additionally, interval funds are priced daily at net asset value although the funds aren't listed on any exchange. Thus, unlike traditional closed-end funds, interval funds don't trade above or below NAV.

"Interval funds are gaining popularity due to their ability to participate in sophisticated strategies, in a safe vehicle," says Reifler.

It's the illiquidity factor that gives interval funds the ability to participate in those unique strategies. "Most interval funds are focused on debt or loan opportunists, stickier investments, aiming to yield a higher return than equity focused strategies," explains Reifler. "They also allow non-accredited investors to participate in a portfolio of sophisticated opportunities, usually reserved for the wealthy or accredited investor."

Reifler says that if the opportunities are structured correctly to mitigate the inherent risks in higher yielding investments, non-accredited investors can "diversify their portfolios, and enjoy a steady stream of income" with interval funds.

"As interval funds have the same public oversight as any public company, they are a safe investment for any financial advisor or independent investor," he adds. "But due to the sophistication of investments, third-party valuation of securities is crucial to maintain transparency."

Unlike most open-end mutual funds, interval funds give investors access to a portfolio of sophisticated investments that are non-correlated to the equity markets, and provide the safety and transparency of any public company, while aiming to outperform mutual funds.

It's a bit of a two-edged sword with interval funds, Reifler explains. On the one hand, illiquidity is what allows investors to participate in higher yielding, risk mitigated, investment opportunities. On the other hand, investors have to "endure" the illiquidity of interval funds for the investment to succeed. "If you can do that, interval fund can be a great addition to an investor's portfolio," he says.

Other financial experts say that interval funds as having the same concept as a mutual fund, with a big caveat.

"They generally only allow investors the chance to sell on a quarterly basis, or sometimes on a monthly basis," says Adam Jordan, director of investment research and management at Paul Ried Financial Group, LLC, in Bellevue, Wash. "While this structure can be O.K. for the right investor, they should not require complete access to their funds. Even for investors that are O.K. with only having access to their funds on a quarterly basis, they should understand it does not mean complete access to their funds."

Jordan says these "open periods" generally have restrictions on the total amount that investors can redeem. "A common threshold is 5% of total fund assets," he says. "This means that if 10% of investors are requesting their funds out -- representing 10% of the total fund assets -- their requests will often be pro-rated, meaning they will likely only get 50% of their funds that they requested."

Many companies running these funds downplay this risk, Jordan adds but it's a very real possibility, especially during times of market stress. "Consequently, investors should understand this going in," he notes. "With that said, interval funds are a step in the right direction from the extremely limited liquidity offered by many non-traded products, such as non-traded REITs."

There aren't a ton of internal funds active right now. While Morningstar doesn't track interval funds, one study - from Robert A. Stanger & Co. - numbered 27 interval funds in operation through the third-quarter of 2016, with an estimated value of $8.7 billion (with more than a dozen more coming down the pike).

Expect those numbers to rise even more, as investors (and their advisors) take a closer look at truly unique interval funds.