The actor passed away Wednesday due to some sort of illness that took his life quickly, according to his ex-wife Nadine Kijner. It's unclear what ailment he might've been battling -- the rest of the details surrounding his death weren't immediately disclosed.

Peter will be best remembered for his recurring role on "Seinfeld" ...  his character "Crazy" Joe Davola appears in 5 episodes in season 4 ... with Joe terrorizing, stalking and threatening Jerry Seinfeld and even dating Elaine at one point.

Aside from his "Seinfeld" fame, Peter acted in movies like"My Dog Skip," "Natural Born Killers," "The Blob," "Se7en" and "Rising Sun," "Born on the Fourth of July" ... just to name a few. He did a lot more work in television, actually ... having made appearances on some big-time shows over the years.

In addition to 'Seinfeld, Peter starred in the TV miniseries "House of Frankenstein" ... as well as guest starring roles on TV shows like "NYPD Blue," "Walker, Texas Ranger," "Diagnosis Murder," "Law & Order," "Perfect Strangers," "Spenser: For Hire," "L.A. Law," "L.A. Firefighters" and lots of others.

All in all, he had 35 acting credits to his name ... and it looks like he left showbiz around 2000.

Peter's ex-wife tells us ... "He was the kindest most caring, giving, considerate man. He was loved by everybody, generous and never had anything bad to say about anyone."

His friend Bill Stetz remembers Peter as "a gentle and loyal friend of soft words and expressive work as an actor and a writer." Peter was 71.