Chelek (Hebrew חלק): "Part." 1/1080 of Sha'ah. = 76 Rega.
Et (Hebrew עת, pl. Etim): "Times." 1/24 Onah. = 1 7/8 Chelek.
Small Onah (Hebrew עונה, pl. Onos): "Period" of 1/24 of Sha'ah. = 24 Etim.
K'dei Achilas Pras (Hebrew כדי אכילת פרס): Time to eat 3/4 of a loaf of bread. Opinions range from 2 minutes to 11 minutes and 9 seconds.
Hiluch Mil (Hebrew הלוך מיל): Time to walk 1 Mil. Opinions range from 18 minutes to 24 minutes.
Sha'ah (Hebrew שעה, pl. Sha'os): An hour. = 24 Small Ona'os.
Onah (Hebrew עונה, pl. Onos): "Period" of daytime or nightime. 1/2 of a day. About 12 Sha'os.
Yom (Hebrew יום, pl. Yamim): A day. = 2 Ona'os
Shavua (Hebrew שבוע, pl. Shavuos): A week. = 7 days.
Chodesh (Hebrew חודש, pl. Chadashim): A month. 29 or 30 days. About 4.214 weeks.
Tekufah (Hebrew תקופה, pl. Tekufos): A quarter / season. "Turn of the sun." About 3 months.
Shanah (Hebrew שנה, pl. Shanim): A lunar year. = 4 Tekufos.
Shmittah (Hebrew שמיטה, Shmittos): Sabbatical Cycle. = 7 lunar years.
Yovel (Hebrew יובל, pl. Yovelim): Jubilee. = 50 lunar years.
Sources
1 Naeh, Rabbi Avraham Chaim, 1947. Shiurei Torah 3:25 pp. 249-250 writes that the etzba is 2 cm,
making an amah equal 48 cm and when measuring an upper or lower bound, it is best to be stringent and use 47 cm or 49 cm per amah, depending on which direction you are measuring.
Since an etzbah is 2 cm, this makes an etzbah meruba'as (square etzbah) equal to 4 cm2.
2 The Aruch Hashulchan YD 286:21 sets 4 amot as one Russian sazhen (7 feet). Dr. Gideon
Freedman proves Aruch Hashulchan YD 201:3 and
Mishna Brurah 358:7 hold that the amah is 21 inches. The Aruch Hashulchan defines the amah as three-quarters of a 71.12 cm
arshin, while Mishna Brurah correlates 53 Russian arshins with 70.83 amot.
Source.
3 Feinstein, Rabbi Moshe. Iggerot Moshe, 1964. OC 1:136 p. 228 writes that the amah is 21.25 inches and one can be stringent and use
23 inches.
4 Kanievsky, Rabbi Yaakov Y. quoting Chazon Ish (Avrohom Yeshaya Karelitz). Shiurin Shel Torah. p. 3.
6 Naeh, Rabbi Avraham Chaim, 1947. Shiurei Torah 3:11 pp. 193 writes that a
kezayis for mitvos de'oraisa is 28.8 cm3, however, in Shiurei Tzion, 1949. p. 70
he writes that it is 27 cm3. Vezos HaBracha (Birur Halacha 1) explains that he retracted from his original ruling.
Source.
7 Vezot HaBracha (Birur Halacha 1) quotes Haggadat Kol Dodi which quotes Rav Moshe Feinstein as saying that a Kezayit is 31.2 cm3.
Source.
8 Kanievsky, Rabbi Yaakov Y. quoting Chazon Ish (Avrohom Yeshaya Karelitz). Shiurin Shel Torah. second ed, pp. 65-66. He writes that the kezayis is 2/3 of a modern egg
which today measures 45-50 grams. The kezayis would therefore be 30-33.333 grams. Vezot HaBracha (Birur Halacha 1:3) records this opinion of the Chazon Ish as 33.3 cm3.
Source.
10 Karo, Yosef. Shulchan Aruch. OC 368 states the opinions that 18 grogeros equals 6 or 8
beitzim.
11 Karo, Yosef. Shulchan Aruch. OC 486:1 states that 2 kezaysim is like 1 beitzah.
12 Karo, Yosef. Shulchan Aruch. OC 612:1 states that a Koseves HaGasah (large date) is a
little less than a beitzah.
13 Karo, Yosef. Shulchan Aruch. OC 612:4 states that some say a pras is 4 beitzim, while
others say it is 3 beitzim.
14 Karo, Yosef. Shulchan Aruch. CM 88:1 states that the calculation of a perutah is the weight
of half a barley of clean silver and the calculation of two ma'ah is the weight of 32 barleys of clean silver.
15 "Rambam" on Mishnah Bechoros 88:46.
16 "Rashi" on Shemos 21:32 states the shekel is weighed as four gold dinar coins, which is half an ounce
according to the correct weight of Cologne.