The Torah in the Book of Vayikra (21:7) lists three kinds of women whom a Kohen is forbidden to marry – a "Gerusha" (divorcee), a "Halala" and a "Zona." Today’s discussion will focus on the precise definition of the term "Zona."
The Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in Hilchot Isureh Bi’a (18:1), discusses the prohibition for a Kohen to marry a "Zona" and presents his definition of this term (listen to audio recording for precise citation). He writes that the category of "Zona" includes several different kinds of women. First, it includes non-Jewish women. A Kohen may not marry a woman who was not born Jewish, even if she later converted. Secondly, the Rambam writes, the term "Zona" includes "a Jewish girl who had relations with a person to whom she is forbidden to marry by force of a prohibition that applies to all." This means that if a woman had previously been married to (or had relations with) somebody whom a Jewish woman may not marry, she becomes ineligible for marriage to a Kohen. Thus, for example, a woman who had married a Mamzer – who is forbidden for marriage – may no longer marry a Kohen, even if she is widowed. This applies also if she had married somebody from the nations of Egypt, Amon or Moab, whom we are forbidden to marry, or if she married a Pesu’a Daka (man with an injury to his reproductive organ that renders him ineligible for marriage). In all these cases, the forbidden marriage renders the woman forbidden to marry a Kohen, as she falls under the category of "Zona."
If a woman had entered into a marriage that was forbidden only under those particular circumstances, but the man himself is eligible to marry other Jewish women, then she does not become a "Zona." If the forbidden marriage is limited to a Kohen, then the lady does not become a "Zona." For example if a Kohen marries a divorcee, she does not be become a "Zona." The reason being that a divorcee is only forbidden to a Kohen but is permissible to all other men. A woman becomes a "Zona" only if – in the Rambam’s words – she married somebody "to whom she is forbidden to marry by force of a prohibition that applies to all." Of course, this woman in any event is forbidden from marrying a Kohen, since she is a divorcee, and, furthermore, her forbidden marriage to a Kohen brings upon her the status of a "Halala." But she does not fall under the category of "Zona," since she had entered into a forbidden relationship that violated a prohibition that applies only to Kohanim.
If the forbidden marriage us limited to a kohen then the lady does not become a zona.
For example if a kohen marries a divorcee, she dies not be become a zona as well. The reason being that a divorcee is only forbidden to a kohen but is permissible to all other men. In the rambams words etc.
If a woman had been in a permissible marriage, but engaged in relations while she was a Nidda, is she considered a "Zona" since she had engaged in a forbidden relationship?
From the Rambam’s careful formulation in defining the term "Zona," it emerges that a woman in this case does not become forbidden from marrying a Kohen. The Rambam writes that a woman becomes a "Zona" if she "had relations with a person to whom she is forbidden to marry." It is clear, according to the Rambam, that if a woman had forbidden relations with somebody she is allowed to marry, then she does not become a "Zona." Thus, even though the woman certainly committed a grave transgression by having relations during her period of Nidda, she does not become forbidden to marry a Kohen, since she engaged in relations with a man whom Jewish women are allowed to marry.
Summary: One of the types of women whom a Kohen may not marry according to Torah law is a "Zona." This is defined as a woman who is not Jewish or who was born non-Jewish, or a woman who engaged in relations with somebody whom Jewish women are not permitted to marry.