Marit ayin
Mar'it-ayin (Mish.: מַרְאִית הָעַיִן; Modern  'appearance to the eye'; Ashkenazic transliteration: maris ayin), is a concept in halakha (Jewish law) which states that certain actions which might  to observers to be in violation of Jewish law, but in reality are fully permissible, are themselves not allowed due to rabbinic enactments that were put in place to prevent onlookers from arriving at a false conclusion. For example, according to the Torah law, the blood of an animal is forbidden to eat, but the blood of a fish is permissible. However, according to the principle of marit ayin, it is forbidden to eat the blood of fish as an onlooker may believe the blood being eaten is from an animal, and may thus believe that animal blood is allowed to be eaten.

According to Ovadia Yosef, who was chief Sephardic rabbi of Israel, marit ayin applies only when the onlooker may mistakenly interpret the action as violating a law given in the Torah.

In private 
There is a disagreement in the Talmud as to whether the concept of marit ayin applies in private. Beit Shamai believes that marit ayin applies even in private, whereas Beit Hillel believes that marit ayin does not apply in private.

Changes over time 
According to the Shulchan Aruch, if something which was prohibited in the times of the Talmud because of marit ayin is no longer a concern due to modern day circumstances, the prohibition is cancelled.

Examples 
Some examples of marit ayin include:

 Eating or drinking kosher food at a non-kosher restaurant
 Killing and eating an animal that is ben pekuah without shechita
 Hanging up wet clothes on Shabbat, since people may think they were washed that day

References 

Jewish law principles
Jewish philosophical concepts
Hebrew words and phrases in Jewish law