What is the definition of "Zonah"?
Rashi: A Zonah is a woman who has relations with a Yisrael to whom she is forbidden. 1
R. Akiva (in Yevamos 61b): A Zonah is literally a prostitute. 2
R. Eliezer (in Yevamos 61b): She is one who had Bi'as Zenus (outside of Kidushin).
The Chachamim (in Yevamos 61b): She is a convert, freed slave, or one who had relations with a man who is forbidden to her. 3
R. Masya ben Charash (in Yevamos 61b): Even if a Sotah had relations with her husband before drinking the water, she became a Zonah.
Rashi: Such as Chayvei K'risos, a Nasin or a Mamzer.
Rashi (in Yevamos, 61b): She strays (she is open to have relations with any man - PF). Also Targum Onkelos and Targum Yonasan translate it like this.
Rashi (in Yevamos 15b): Chayavei Kerisus, or Chayvei Lavin of a Zar me'Ikara (she was forbidden to him from birth, but not a Yevamah le'Shuk or a dovorced woman whose husband took her back after she was married to someone els1e). Ritva (in Yevamos 70a) - an Isur to all of Yisrael, such as a Petzu'a Daka, even if he is not a Zar me'Ikara. See also Torah Temimah, who cites only this opinion and elaborates at length.
What is the definition of "Chalalah"?
Rashi, Rashbam and Targum Yonasan: A Chalalah is the daughter of Pesulei Kehunah 1 (and the woman herself after she has had relations with someone Pasul to Kehunah - even if the P'sul is only an Asei - See Torah Temimah on Pasuk 14, citing Yevamos, 60a and notes 67 & 68.
Rashi: A divorcee, a Chalutzah (makes a Chalalah mi'de'Rabanan) with a Kohen Hedyot, or a widow with a Kohen Gadol. See also Rashbam.
What are the connotations of "Lo Yikachu"?
Yevamos, 54a: We learn via a Gezeirah Shavah 'Kichah' 'Kichah' from Chayvei Kerisos in Acharei-Mos, 18:18 "ve'Ishah el Achosah Lo Sikach" 1 - that the Kohen is Chayav even for Ha'ara'ah (being intimate without actually penetrating).
See Torah Temimah, note 47.
What are the implications of the phrase "Ki Kadosh hu l'Elokav"?
Seforno: All the offspring of a Kohen - who is termed "Ba'al be'Amav" - are sanctified to his G-d. Consequently, in the event that they marry one of these women, they desecrate, not only their own honor, but also that of their children, who will not be worthy of that Kedushah.
What is the difference between a Gerushah - whom a Kohen is forbidden to marry, and an Almanah - whom he is permmited to marry?
Oznayim la'Torah #1: The Kohen is forbidden to mary a Gerushah, because, as the Rashbam explains in Pesachim, 112, during Tashmish, she thinks about her first husband and the children are close to being Mamzerim - close enough to place a stain on the Kedushah of a Kohen. 1
Oznayim la'Torah #2: The prohibition against marrying a Gerushah is based on the Pasuk in Ki Seitzei, 24:4 "Acharei asher Hutama'ah." Consequently, since the a Gerushah possesses an element of Tum'ah, 2 the Torah forbids a Kohen to marry her. 3
Which is not the case regarding an Almanah on whom there is a decree to foet her first husband (Oznayim la'Torah),
Oznayim la'Torah: Albeit only a Re'ach Tum'ah - Refer to 21:7:151:2 and note.
See Oznayim la'Torah DH 've'Ishah Gerushah me'Iyshah #1 who elaborates.
Why does the Torah insert the (otherwise superfluous) word "Gerushah me'Iyshah"?
Rashi (in Yevamos, 94a) and Moshav Zekenim #2 (citing Gitin, 82b): If a man divorces his wife and states 'You are divorced from me, but not permitted to other men', she is disqualified from the Kehunah, 1 even after her husband dies, in spite of the fact that she was divorced only from her husband (and not permitted to others). 2
Moshav Zekenim #1 (citing the Sifra): If a Kohen went overseas, and witnesses [mistakenly] told his wife that he died, and another man is Mekadesh her; in the event that her husband returns, she is permitted to marry a Kohen even if the latter divorced her, since she was not divorced from her real husband. 3