Etymology
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omega (n.)

final letter of the Greek alphabet, c. 1400, from Medieval Greek omega, from classical Greek o mega "big 'o' " (in contrast to o micron "little 'o' "); so called because the vowel was long in ancient Greek. From o + megas "great, large, vast, big, high, tall; mighty, important," from PIE root *meg- "great." Used figuratively for "the last, the final" of anything (as in Revelation i.8) from 1520s.

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alpha (n.)
c. 1300, from Latin alpha, from Greek alpha, from Hebrew or Phoenician aleph (see aleph). The Greeks added -a because Greek words cannot end in most consonants. Sense of "beginning of anything" is from late 14c., often paired with omega (the last letter in the Greek alphabet, representing "the end"); sense of "first in a sequence" is from 1620s. In astronomy, the designation of the brightest star of each constellation (the use of Greek letters in star names began with Bayer's atlas in 1603). Alpha male was in use by c. 1960 among scientists studying animals; applied to humans in society from c. 1992.
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a- (3)
prefix meaning "not, without," from Greek a-, an- "not" (the "alpha privative"), from PIE root *ne- "not" (source also of English un-).

In words from Greek, such as abysmal, adamant, amethyst; also partly nativized as a prefix of negation (asexual, amoral, agnostic). The ancient alpha privatum, denoting want or absence.

Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."
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beta (n.)

second letter of the Greek alphabet, c. 1300, from Greek, from Hebrew/Phoenician beth (see alphabet); used to designate the second of many things. Beta radiation is from 1899 (Rutherford). Beta particle is attested from 1904. Beta male, pejorative term for a risk-avoidant, non-confrontational man perceived as a follower or supporter rather than a leader, is by 2005, transferred from zoology (birds, primates), where it is attested by 1962 (compare alpha male under alpha).

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Vega (n.)
1638, bright northern star, the alpha of Lyra, from Arabic (Al Nasr) al Waqi translated variously as "the eagle of the desert" or "the falling vulture" (or bird).
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Capella 
bright northern star (fifth brightest in the heavens), the alpha of the constellation Auriga, by 17c., from Latin capella, literally "little she-goat" (Greek kinesai kheimonas), diminutive of capra "she-goat," fem. of caper "goat" (see cab).
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omicron (n.)

15th letter of the Greek alphabet, c. 1400, literally "small 'o,' " from o + Greek (s)mikros "small" (see micro-). So called because the vowel was "short" in ancient Greek. Compare omega.

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A 

first letter of the Roman alphabet, based on Greek alpha (see alpha). In music from c. 1600 as the name of the sixth note of the natural scale; it is the note given by a fixed-tone instrument (usually oboe or organ) to which all the instruments of an orchestra are tuned. As a blood type, 1926, denoting A agglutinogens. The A side of a two-sided record (by 1962, see side (n.)) held the material chosen for promotion. A-bomb, short for atom bomb, was in newspaper headlines by Aug. 8, 1945.

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sorority (n.)
1530s, "a society of women, body of women united for some purpose," from Medieval Latin sororitas "sisterhood, of or pertaining to sisters," from Latin soror "sister" (see sister). Sense of "women's society in a college or university" attested by 1887 (Alpha Delta Pi claims founding in 1851).
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Castor 

one of the divine twins (brother of Pollux), also the name of the alpha star of Gemini, Latin, from Greek Kastor, perhaps literally "he who excels." They were sons of Tyndarus, king of Sparta (but in post-Homeric myth of Zeus in the form of a swan)  and Leda.

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