A character who fits "Jewish" stereotypes, possibly played by an actor who is Jewish in real life, but who is never (or cryptically) actually stated to be a Jew on the show. In some cases, the show might present contradictory evidence on both sides. Often this is because it's difficult to establish a character as Jewish without depicting them as a walking stereotype, particularly when there's no omniscient narrator to simply say so or when they don't fit the stereotype.
Note that having Yiddish as a Second Language is a clue but is not conclusive. As Lenny Bruce said, all New Yorkers are at least a little Jewish. Having a "Jewish" name ending in -berg, -stein or the like is also evidence but not proof. Most of these names are actually just common German names that many Ashkenazi Jews took when required to adopt some type of surname. Authentically Hebrew names will often begin with "Bar" or "Ben," so they're a good tip-off. Being named "Cohen" or "Cahn" makes it nearly certain. For Sephardic Jews, a common name is "Garcia", but it's also a common Hispanic name that by itself doesn't prove anything.
In addition, such a character would often demonstrate "stereotypically Jewish" appearance and personality traits: those include being a Brainy Brunette, a love for books and the written word in general, and a sarcastic wit often accompanied by self-irony. In the past, Ambiguously Jewish characters were often depicted as greedy and cunning, but due to the decidedly antisemitic nature of the stereotype, today's writers either ignore or at least downplay it.
If the series is set in New York City or some equivalent, you can usually expect at least one of these. About 12% of New Yorkers are Ashkenazi Jewish, and they've had a great deal of impact on the culture.
This is the exact opposite of Informed Judaism, which is when a Jewish character's faith is plainly stated, usually as an afterthought, but never really exhibited by their thoughts or actions and has no relevance in the plot.
If a character is non-human and displays stereotypical Jewish behavior, it's Space Jews.
