(chemistry) A rule stating that atoms lose, gain, or share electrons in order to have a full valence shell of 8 electrons. (Hydrogen is excluded because it can hold a maximum of 2 electrons in its valence shell.)

Containing eight syllables

(dated or historical, now offensive) Someone having one-eighth black ancestry. / (dated or historical, now offensive) Someone having 1/64th black ancestry: the child of a quintoon and a white man.

(meteorology) The proportion of the sky that is obscured by clouds, in eighths (one octa means that one eighth of the sky is obscured, two octas that one quarter is obscured, and so on).

(music) The octave that begins on the C that is two ledger lines below the staff of the bass clef

(music) The octave directly below the great octave.

(paper) A paper size, (2.5"-3.1" x 2.5"-3.3"), one forty-eighth of a standard uncut flat sheet. / (printing) A book size, (2.5"-3.1" x 2.5"-3.3"), corresponding to the paper size.

(organic chemistry) Any of very many isomers of the aliphatic hydrocarbon having 28 carbon atoms, but especially n-octacosane CH₃(CH₂)₂₆CH₃

plural of octane number

An octane number, used by the American Society for Testing and Materials to estimate the antiknock characteristics of a fuel at low engine speeds