Which album, in your view, is the "definitive" album of the 1990's. It doesn't have to be your personal favorite album released that decade, you can even dislike or hate it. By "definitive," I'm talking sales, cultural impact, legacy, etc.
According to this list the soundtrack to The Bodyguard is the biggest selling album of the 1990's. I wouldn't rank it as the "Definitive" album of that decade, however, as apart from "I Will Always Love You" I don't remember any other song from it. Yes, it sold a ton of albums, but seems more like a relic from the early 1990's than an album that captured the zeitgeist of that decade.
I'm tempted to go with Nirvana's Nevermind, which, and sorry to sound so cliche, really was a game-changer that influenced rock music/fashion for the decade, as well as being one of the decade's biggest sellers (10 million sold in U.S.). Pearl Jam's Ten would be another contender.
Many people on this board might pick The Downward Spiral or The Fragile, while I would pick Soundgarden's Superunknown as my personal favorite album from the 1990's. But the "definitive" album? One choice really stand out for me--Jagged Little Pill by Alanis Morissette.
Can you understate how frigging huge that album was? I didn't even own that album when it was massively popular but you couldn't escape from it. The songs--"You Outta Know," "You Learn" and especially "Ironic" were everywhere. And this was an album that appealed to all kinds of people. You would hear her songs alongside Pearl Jam and The Smashing Pumpkins on rock-oriented stations, but she was also played on soft-rock, college, and adult-contemporary radio too. At Applebees, the mall, Trader Joe's--you couldn't escape those songs.
In addition to the huge singles and massive sales, the album just sort of screams "1990's," in all it's angsty, Generation X glory. So while Jagged Little Pill isn't my personal favorite album of the 1990's, it gets my vote as the "definitive' album from the 20th century's final decade.