What are zero-day options? Here's how they're taking over a key corner of the stock market and why they could pose a big risk.

• It's been one year since the CBOE launched zero-day options contracts and they're starting to take over the stock market.
• Zero-day options expire the same day they are issued and they now make up 50% of S&P 500 options activity.
• A new ETF has launched utilizing the options contracts, and with enough scale they could jolt the stock market in a big way.

There's a new options trading product that is taking over Wall Street, and it could ultimately pose a big risk for the stock market as it gains in scale.