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Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (cf. Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- "on" (cf. Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from"). Also used in Old English in many places where we would now use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877. On time is from 1890.
A subatomic particle: neutron.
A unit: photon.
A basic hereditary unit: codon.
Inert gas: radon.
A chemical compound that is not a ketone or a compound that contains oxygen in a carbonyl group: parathion.
light; the sun, (Gen. 41:45, 50), the great seat of sun-worship, called also Bethshemesh (Jer. 43:13) and Aven (Ezek. 30:17), stood on the east bank of the Nile, a few miles north of Memphis, and near Cairo, in the north-east. The Vulgate and the LXX. Versions have "Heliopolis" ("city of the sun") instead of On in Genesis and of Aven in Ezekiel. The "city of destruction" Isaiah speaks of (19:18, marg. "of Heres;" Heb. 'Ir-ha-heres, which some MSS. read Ir-ha-heres, i.e., "city of the sun") may be the name given to On, the prophecy being that the time will come when that city which was known as the "city of the sun-god" shall become the "city of destruction" of the sun-god, i.e., when idolatry shall cease, and the worship of the true God be established. In ancient times this city was full of obelisks dedicated to the sun. Of these only one now remains standing. "Cleopatra's Needle" was one of those which stood in this city in front of the Temple of Tum, i.e., "the sun." It is now erected on the Thames Embankment, London. "It was at On that Joseph wooed and won the dark-skinned Asenath, the daughter of the high priest of its great temple." This was a noted university town, and here Moses gained his acquaintance with "all the wisdom of the Egyptians."