I was learning with a Rabbi today and we came across the following question. An Almana, widow, is not allowed to be married to a Kohen Gadol (see Mishna Yevamos 9:2). However, what defines a widow? Meaning, if the woman was married, had biah with her husband, and then her husband dies, it's clear that because she is no longer a virgin, she can't marry the Kohen Gadol. But suppose the man dies without having biah with his wife: Do we say that this woman cannot marry a Kohen Gadol because she is an Almana(in the simple sense that she was married then her husband die), or is it necessary that she have had biah with her husband before he died that would label her an Almana and ultimately, unable to marry the Kohen Gadol?
The mishna in Y'vamos (6:4) indicates that a widow is forbidden to a Kohein Gadol whether she was a widow only from erusin (when intimacy was still forbidden) or whether she was a widow even from nisu'in:
כוהן גדול לא יישא את האלמנה--בין אלמנה מן האירוסין בין אלמנה מן הנשואין.
This is quoted as halacha by the Rambam (Hil. Isurei Bi'ah 17:11):
אלמנה בין מן האירוסין בין מן הנישואין אסורה.
Both types of widow are biblically forbidden to a Kohein Gadol. A widow from either erusin or nisu'in is forbidden to a Kohein Gadol due to a negative prohibition (Vayikra 21:14, אַלְמָנָה... לֹא יִקָּח), and a widow from nisu'in is additionally forbidden due to a positive commandment (Vayikra 21:13, וְהוּא אִשָּׁה בִבְתוּלֶיהָ יִקָּח; Sanhedrin 18b-19a; Rashi ibid. 18b, s.v. ka pasik v'tani; Sefer HaChinuch §273, or §294 in some versions).
The gemara indicates that the definition of almana (widow) for this purpose should obviously include an almana from erusin (Y'vamos 59a; Rashi ad loc., s.v. p'shita). However, since the prime example of an almana in the Pentateuch is Tamar (B'reishis 38:11), who was a widow from nisu'in, the mishna had to clarify that the term almana indeed includes even widows from erusin.
The gemara comments that, notwithstanding the example of Tamar, the parameters of the almana status vis-à-vis a Kohein Gadol are derived from the parameters of the g'rusha (divorcee) status vis-à-vis a Kohein; just as "g'rusha" is defined to include divorcees from erusin or nisu'in, so is "almana" defined to include widows from erusin or nisu'in (Y'vamos 59a; see Rashi ad loc. s.v. dumya d'grusha).
From a drush perspective, the gemara in Kesuvos 10b expounds upon the meaning of the word "almanah":
מאי אלמנה אמר רב חנא בגדתאה אלמנה על שם מנה
Why [is a widow called an] almanah? Rav Ḥana of Baghdad said: [A widow is called an] almanah after the maneh
Namely, "almanah" is an acronym for "al shem maneh"- ie the value of a widow's Kesuvah is worth a "maneh" (one hundred dinars).