Short answers, pulled from the story.
A mashup is a song created by blending two or more prerecorded songs, typically by layering one song's vocal track over another song's instrumental track and adjusting tempo and key where necessary. Because it recombines existing recordings into something new, a mashup can be treated as a transformative work, and in the United States that can shield it from copyright claims under the fair use doctrine.
Harry Nilsson is described by some as the inventor of the mashup, based on a track on his 1967 album Pandemonium Shadow Show billed as a cover of the Beatles' "You Can't Do That." The recording actually wove his own vocal recreations of more than a dozen Beatles songs into that single track.
"Dub Be Good to Me," released by Norman Cook's act Beats International in 1990, is described as the first mashup to reach significant mainstream success. It reached number one on the UK Singles Chart by combining re-recorded vocals of the SOS Band's "Just Be Good to Me" with the Clash's "The Guns of Brixton."
Producer Richard X built a 2001 bootleg mashup called "We Don't Give a Damn About Our Friends," combining Adina Howard's "Freak Like Me" with Tubeway Army's "Are Friends Electric." Unable to secure the original vocals for commercial release, he had the Sugababes re-record them, and the resulting single, released in April 2002, gave the group their first UK number one.
Kylie Minogue performed a mash-up of "Can't Get You Out of My Head" with New Order's "Blue Monday" at the 2002 Brit Awards, held on the 20th of February 2002. Titled "Can't Get Blue Monday Out of My Head," it is cited as one of the first times a mainstream artist used a mashup in a live performance.
DJ Hero, a 2009 rhythm game from Activision, includes more than 90 pre-made mashups played on a turntable controller. Fuser, a 2020 game from Harmonix, lets players build mashups from more than 100 songs using instrument stems, and Fortnite Festival added mashup creation across more than 100 songs starting in December 2023.