from The American Heritage® Dictionary of the English Language, 4th Edition
- adj. Of or relating to the position of two celestial bodies when they are 60° apart.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Of or noting the aspect or position of any two celestial bodies separated by 60°.
- n. The specific term used for a quantile of six equal proportions.
- n. statistics Any of the subsets thus obtained.
- n. A segment that is a sixth of the whole.
- n. A sextile aspect or position.
from the GNU version of the Collaborative International Dictionary of English
- adj. Measured by sixty degrees; fixed or indicated by a distance of sixty degrees.
- n. The aspect or position of two planets when distant from each other sixty degrees, or two signs. This position is marked thus: ✶.
from The Century Dictionary and Cyclopedia
- In astrology, noting the aspect or position of two planets when distant from each other sixty degrees or two signs.
- n. The aspect of two planets, or of the sun and a planet, when they are at a distance of 60 degrees from each other.
You can follow any responses to this entry through the One Response to "The most mentions of" sextile "you'll find on the internet" art vandelay says:
That there was this triple configuration on that day ” namely the sextile of Saturn and the Sun, the sextile of Mars and Jupiter, and the quadrature of Mercury and Mars, is proved by the change of weather; for after a frost of some days, that very day became warmer, there was a thaw and a fall of rain.”
The detail of the inconsistencies in the etymological treatment and in the alloseme/lemma status of the quintile and sextile that you mention is no less diabolical!
The Universe supports your decision with a stellar sextile, the aspect of opportunity, from Venus to Jupiter on the 2nd.
Dividing the zodiacal wheel by 3 (trine 120 degrees) and 6 (sextile 60 degrees)a harmonious relationship.
The four planets are each linked to the next one by a 60-degree angle sextile aspect.
Antinous, Nero Sporus, &c. The physicians refer this to their temperament, astrologers to trine and sextile aspects, or opposite of their several ascendants, lords of their genitures, love and hatred of planets; 
I tend to restrict aspect analysis to the following major five angles: conjunction, sextile one of my favs, square, trine and opposition.
Aspects are of four varie - ties: opposition (to the seventh sign) and quartile (to the fourth) are generally bad, and trine (to the fifth) and sextile (to the third) good.
Blemishes in the eyes, or a peculiar weakness in the sight, is the result of her being afflicted by the Sun. Her conjunction, semi-sextile, sextile, or trine, to Jupiter, is exceeding fortunate; and she is said by the old Astrologers to govern the