I'd say there wasn't one that defined an era they're was a bunch that created the era.
You couldn't say one without an arguement for another 3 or 4.
The Attitude Era Officially started at King of the Ring 1996 with Austin's 316 speech, but the turning point was the MSG/Kliq deal, had that never of went down HHH would of won KOTR 96 as slated to, so Austin mightn't have won a KOTR had HHH of won the 1996 edition.
November 1996 Pillman-Austin gun showdown, that to me was the breaking of the glass ceiling, I consider Austin/Hart I (survivor series 96) to be the last of the classic old school type of matches, yet showed hints of the Attitude era style around the ring brawls, but the Pillman/Austin home invasion gun shot REALLY pushed the buttom that WWE was in for a radical change.
Survivor series 1997, screw job, people will probably argue that this isn't significant towards the Attitude era, Bret was old school, even his feuds had old school roots in them, DX was the new band, the band that broke rules, and pushed smut tv on fans, now DX were the reason people watched WWF back in 1997-98 because they were cool (more so the 98 version) pushed more limits as far as other wrestling promotions goes, but had Bret not have been screwed we'd never have had Mr Mcmahon characher and Austin/Vince.
Wrestlemania 14, This was the event I consider Attitude Era's greatest PPV, this was one of them wrestlemania's you really WANTED and HAD to watch previous Mania's had lacked that Wrestlemania feel but this one felt special and you had to be apart of it, plus the passing of the torch Austin/HBK.
Vince Buying WCW that for me was the end of the Attitude era, he not competition, and the product suffered, but they were the significant events that shaped the attitude era for me personally.
Honorable mentions, Summerslam 2000, Austin getting knocked over and the Hardcore title all contributed to the attitude era for me, summerslam had some classic matches, austin getting knocked over was pretty gnarly and unexpected for me anyways, and the Hardcore title WAS the belt to have in the 1998-2001 era, in 2002 it took a dive after Undertaker dropped it to Maven yes MAVEN.