What is the precise definition of Maris Ayin, and to what cases does it apply? (i.e. what are the parameters for applying it)
Maris Ayin (literally "the vision of the eye") describes rabbinic enactments that were put into place to prevent a third-party viewing one's actions from arriving at the incorrect conclusion that a forbidden action is permitted.
- It is forbidden to eat the blood of fish (which itself is permitted according to Torah law) lest someone watching you think that either you were eating the blood of an animal (something which is forbidden), and therefore it is permitted to eat the blood of an animal, or think that you are committing a sin. (Keritut 21b)
- If you own a bath house, you are not permitted to rent it out to a non-Jew who will run it on Shabbat, lest people (who did not know that you rented it out) think that you are performing prohibited actions on Shabbat
- A Jew who is a resident of Israel, who is traveling outside of Israel for one of the festivals should not perform prohibited actions on the second day of the festival (which is only for those who are not residents of Israel) because someone may see this being done and, not knowing that this person is a resident of Israel, would assume that either a prohibited action is being performed or that the prohibited action is permitted. (There are other reasons as well - this halacha is a point of contention among modern poskim - if it applies to you, please make your own inquiries and do not rely on this site)
It has many applications across halacha, including the laws of Shabbat, Avodah Zarah and Kashrut.
An important additional principle is that כל מקום שאסרו חכמים מפני מראית העין - אפילו בחדרי חדרים אסור - Anything that the sages prohibited because of maris ayin is forbidden, even when it is performed in a private room (Beizah 9a). In other words, once something is prohibited by chazal for this reason, one is not allowed to rationalize that one can perform the action in private because no one will see it. (This principle is also the subject of much debate among halachic authorities).