On "Seinfeld," George Costanza's fiancée who dies after licking toxic wedding envelopes is Susan Ross. Susan is George's on-again, off-again girlfriend and later becomes his fiancée during the show's seventh season.
The fatal incident occurs in "The Invitations," the season-seven finale. George and Susan are preparing for their wedding, and George buys inexpensive wedding invitations. The envelopes contain a toxic adhesive, and Susan repeatedly licks them while addressing the invitations. She becomes ill and dies, leaving George unexpectedly single and free from the marriage he had repeatedly tried to escape.
The storyline is deliberately built around George's conflicted attitude toward commitment. He had proposed partly because he felt trapped by the relationship, then spent much of the engagement searching for ways out. Susan's death turns that comic anxiety into one of the series' darkest jokes: George is devastated publicly but privately realizes that the wedding will not happen.
Susan is not Marla Penny, Jeannie Steinman, or Sidra Holland. Those names belong to other women connected to the show's romantic storylines, but none is George's ill-fated fiancée.