President Donald Trump and North Korean leader Kim Jong Un signed a document on Tuesday stating that Pyongyang would work toward "complete denuclearization of the Korean Peninsula."

The historic agreement came after the two leaders held several meetings throughout the day.

"Fox News Sunday" host Chris Wallace said one of the key issues will be how "denuclearization" is defined.

"The U.S. has made it very clear what they think, which is that complete, verifiable, irreversible denuclearization of the North Korean weapons program, both its nukes, its enrichment and also its missiles," Wallace said on "Fox News @ Night." "Now what does Kim mean by that?"

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He wondered if Kim will insist that the U.S. remove its "nuclear umbrella" from the region.

Wallace said another key issue will be when economic sanctions are lifted, since Trump has said they will remain in place until North Korea completely dismantles its nuclear program, while Kim has pushed for a phased process.

"That's a big difference," Wallace said. "And my guess is that's going to be one of the key sticking points in working this out."

Watch more from "Fox News @ Night" above, and stay with Fox News Channel for coverage of the historic Singapore summit.

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