English Words Starting With O

• octane

  nounC1
  /ˈɒk.teɪn/A hydrocarbon (C8H18) in the paraffin series, present in petroleum. Specifically refers to a measure of a fuel's ability to resist knocking in an internal combustion engine.
• octane number

  nounC1
  /ˈɒk.teɪn ˌnʌm.bər/A numerical value representing a fuel's resistance to knocking or pinging in an internal combustion engine.
• octanedioic acid

  nounVery Low
  /ˌɒk.teɪn.daɪˌəʊ.ɪk ˈæs.ɪd/A saturated dicarboxylic acid with the formula HOOC−(CH₂)₆−COOH; a white crystalline organic compound used in chemical synthesis.
• octangle

  adjectivenounRare
  /ˈɒkˌtæŋ.ɡəl/A polygon with eight sides and eight angles.
• octanoic acid

  nounVery Low (C2, technical domain)
  /ˌɒk.təˌnəʊ.ɪk ˈæs.ɪd/A saturated fatty acid with eight carbon atoms, naturally occurring in various animal and plant fats and oils.
• octans

  nounVery low (specialist/technical)
  /ˈɒk.tænz/A faint southern constellation containing the south celestial pole.
• octant

  nounC2
  /ˈɒktənt/An instrument for measuring angles, especially one with a graduated arc of 45° or one-eighth of a circle, used historically in navigation and astronomy.
• octarchy

  nounVery rare
  /ˈɒk.tɑː.ki/A government or state ruled by eight persons or a coalition of eight political entities.
• octaroon

  nounVery Low / Archaic
  /ˌɒktəˈruːn/A dated and offensive historical term for a person with one-eighth Black ancestry.
• octateuch

  nounVery rare / Obscure
  /ˈɒktətjuːk/A set of eight books.
• octave

  adjectivenounC1
  /ˈɒk.tɪv/In music, the interval between one musical pitch and another with double its frequency; a set of eight notes in a diatonic scale.
• octave coupler

  nounLow
  /ˈɒktɪv ˈkʌplə/A mechanism on certain keyboard instruments that connects a key to the key an octave above (or below) it, causing both to sound when the lower one is played.
• octavia

  nounRare (as a lexical item; as a name, low frequency outside specific cultural/historical contexts)
  /ɒkˈteɪ.vi.ə/A female given name of Latin origin, historically used by prominent Roman families.
• octavian

  nounC2
  /ɒkˈteɪ.vi.ən/The historical name for Gaius Octavius Thurinus, later known as Augustus, the first Roman Emperor.
• octavius

  nounVery Low
  /ɒkˈteɪ.vɪ.əs/A masculine given name, historically associated with Roman heritage. In ancient history, it was the personal name of the Roman emperor Augustus before his rise.
• octavo

  adjectivenounC2/Rare
  /ɒkˈteɪ.vəʊ/A book size resulting from folding a sheet of paper into eight leaves (sixteen pages).
• octet

  nounC1
  /ɒkˈtet/a group or set of eight
• octillion

  adjectivenounRare
  /ɒkˈtɪl.i.ən/A cardinal number represented by 1 followed by 27 zeros (10^27) in the short scale system, or 1 followed by 48 zeros (10^48) in the long scale system.
• october

  nounA1
  /ɒkˈtəʊbə/The tenth month of the year in the Gregorian calendar, following September and preceding November.
• october revolution

  nounC1
  /ɒkˌtəʊbə ˌrevəˈluːʃn/The 1917 Bolshevik seizure of power in Russia, which led to the establishment of the world's first communist state.

Showing 381–400 of 3187 words.