When jammin’ to a mahjong tune, and singing about its strategy and wait analysis, the hot riffs are all about octaves, man. But what instrument do you play those octaves on?
When thinking about mahjong strategy and wait analysis, much of that science gets published out of Japan, and so logically many of the terms used to describe that science likewise derives out of Japan. In particular, in this article we're talking about 筋 suji sets.*
When translating Japanese mahjong terms to English, suji (筋) is commonly translated to octave, although I can’t explain how this poor translation ever caught on so well. Octave specifically references an interval of eight (either in music or in poetry), and an interval of eight has nothing to do with suji sets.
Suji can also be translated to “piano keys”, and the fact that both translations are musically related is about as close as “octave” can get to being relevant to a suji set. Suji sets happen to have an interval of four, not eight, of course, and that interval is called a quatrain, not an octave. Suji as translated to piano keys at least implies that the tiles complete a defined set.
To keep in tune with suji as it plays to mahjong, if you don't want to use quatrain (which isn't exactly a household word), a chord would be the best translation for suji set. Chords in music are groups of notes that harmonize well together (which aren't spaced eight steps apart), and that is exactly what suji sets are for tiles; tiles that harmonize well together (which likewise aren't spaced eight steps apart either). That, and quite nicely 筋 (suji) as a word can also be translated directly to "cord" anyway, which really makes chord a playful fit. And that's music to my ears.
* Suji sets describe those tiles which are related because they sit on either side of a double-sided wait. For example, 1 and 4 sit on either side of a 23 which waits to complete a chow. As such, 1 and 4 are a suji set. When depicted on a keyboard, suji sets can be represented as a chord.