The Torah in Parashat Emor (Vayikra 21:7) forbids a Kohen from marrying a “Halala” – a woman born from a union between a Kohen and a woman whom Kohanim are forbidden to marry. For example, if a Kohen Gadol marries a widow, or a regular Kohen marries a divorcee, a girl produced from either of these marriages is forbidden for a Kohen to marry. Since she was born from a marriage that is forbidden to Kohanim, she may not marry a Kohen.
A woman also becomes Halala if she has a relationship with a Halal – a Kohen who is disqualified from serving in the Bet Ha’mikdash, such as if he was produced by a marriage forbidden for Kohanim. Even though a Halal is permitted to marry an ordinary woman, nevertheless, the woman he marries becomes a Halala and is thus forbidden from marrying a Kohen.
A Kohen who marries a Halala and consummates the marriage is liable to Malkut for violating this prohibition. However, he does not become a Halal as a result of this relationship.
A woman becomes a Halala by having a relationship with a Halal either “Ke’darkah” (in the normal manner of cohabitation) or “She’lo Ke’darkah” (in an unusual manner). This relationship must have occurred when she was at least three years old, and the Kohen was at least nine years old. If a Halal merely betrothed a woman, she does not become a Halala. If they got married, but never consummated the marriage, then the woman is considered a Halala and may not marry a Kohen; however, a Kohen who marries her is not liable to Malkut.
A woman born from a union between a Kohen and a woman in a state of Nidda is not a Halala, and may marry a Kohen.
A Halusa – a woman whose husband died without children, and who performed the “Halisa” ceremony in lieu of marrying the husband’s brother – is forbidden from marrying a Kohen “Mi’de’rabbanan” (by force of Rabbinic enactment). By the same token, then, if a Kohen marries a Halusa, the daughter produced by this union is forbidden from marrying a Kohen “Mi’de’rabbanan.”
If a Kohen marries somebody in violation of a prohibition that applies to all people, and not just to Kohanim, the daughter produced from this union is not considered a Halala. Thus, for example, if a Kohen has a relationship with his mother’s mother – a relationship forbidden “Mi’de’rabbanan” – and they produce a girl, she is not a Halala. (If a Kohen has a relationship with an immediate family member, then the daughter is, of course, a Mamzeret, and in any event is forbidden from marrying a Kohen.)
In some situations, a woman is “Safek Megureshet” – in a state of uncertainty about her status, as to whether or not she is divorced. This happens in a situation where a woman was betrothed but there is some question as to the validity of the betrothal, and so she received a divorce from the man in case the betrothal had been valid. It is now questionable whether she may marry a Kohen, because it is uncertain whether she is a divorcee, or had never been Halachically married. If this woman then marries a Kohen, the daughter is considered a “Safek Halala,” and may not marry a Kohen due to her uncertain status. If this union between a Kohen and a “Safek Megureshet” produced a boy, then he is a “Safek Halal.” He may not eat Teruma (the hallowed food reserved for valid Kohanim), because he might be a Halal, but he may not come in contact with a human corpse, because he might be a valid Kohen. By the same token, he may not marry a woman forbidden for Kohanim.
A Bat Kohen (daughter of a Kohen) is permitted to marry a Halal. The son of a Halal is also a Halal.
This prohibition applies to Kohanim, in all places and in all times. A Kohen who marries a woman who definitively has the status of Halala is liable to Malkut. If she is questionably a Halala, or a Halala by force of Rabbinic enactment, then he receives Malkut Mi’de’rabbanan.