Definitions

1. adjective

relating to the eighth tone or mode in music

“The octonal scale is a lesser-known musical mode.”

2. noun

a tone or mode in music, especially the eighth one in a series

“The singer’s use of the octonal tone added a unique flavor to the song.”

3. noun

one of a series of eight equal or nearly equal parts

“The octonal division of the country into eight regions has made governance easier.”