Which Seinfeld episode introduced the phrase 'yada yada'?

The Seinfeld episode that introduced the phrase “yada yada” was “The Yada Yada.”

“The Yada Yada” is the 19th episode of season eight and originally aired on April 24, 1997. Its central joke is the way George's new girlfriend uses “yada yada yada” to skip over supposedly important details, leaving George convinced that she is concealing something significant.

The episode did not invent the expression. Versions of “yadda yadda” were already used in American speech, comedy, and earlier television, including by comedian Lenny Bruce. Seinfeld nevertheless gave the phrase a major cultural boost by turning it into a recurring device for summarizing away inconvenient or suspicious parts of a story.

The episode also includes the term “anti-dentite,” Jerry's complaint about prejudice against dentists, and features Bryan Cranston as Tim Whatley. A common wording mistake is to call the episode “Yada Yada Yada”; its official title is the singular “The Yada Yada,” even though the repeated phrase is usually spoken three times.