Sources say the set is on track to sell at least 2.5 million in pure album sales in its first week, and sold more than 900,000 copies alone through the iTunes Store on its first day of release.

"25" was released on November 20 through XL/Columbia Records. It's Adele's third studio album.

*NSYNC currently owns the single largest sales week for an album since Nielsen Music began tracking point-of-sale music purchases in 1991. (Prior to 1991, there was no authoritative music sales tracking service in the U.S. and thus, it was mostly unknown how many copies of an album or song/single were sold in a single week.)

*NSYNC's "No Strings Attached" debuted with 2,416,000 sold in the week ending March 26, 2000. "No Strings Attached" has since remained the only album to sell 2 million copies in the U.S. in a single week.

Nielsen Music's tracking week runs Friday through Thursday each week, so the first week for "25" will conclude on November 26.

Billboard is scheduled to report the album's debut week sales on November 29, once Nielsen has finished processing its weekly data.

Before its release, it was reported that "25" shipped 3.6 million physical copies to retailers.

In addition, sources were forecasting its CD configuration to sell 1.5 million in its first week, while its digital edition could move another 1 million (meaning, its first week might be at least 2.5 million in total).

Now perhaps the question is: How much bigger can "25" get? Will it blow past the 2.5 million threshold? Can it hit 3 million?

And what will "25" sell in its second and third week?

If "25" sells as expected, it will become the 20th album to sell at least a million copies in a single week in the Nielsen era.

It would also instantly be the largest selling album of 2015 in total, surpassing the sales of Taylor Swift's "1989," which has sold 1.74 million this year (through Nov. 12).

"25" is also on course to debut at No. 1 on the Billboard 200 albums chart (dated Dec. 12), which would give the diva her second No. 1.

She first led the list with her last album, "21," which spent 24 nonconsecutive weeks atop the list -- most weeks at No. 1 for an album by a woman.

"21" has sold 11.23 million copies in the U.S., and is the tenth-largest selling album of the Nielsen era.

(CNN)