Have you been skipping like, almost every one of Rin's posts?
It has a translation.
It's not like the Gears, where the person using it is screaming it out ala "GEAR SECONDO!" It's not an ability & can translate to "ambition" and/or "willpower." Therefore, it should be written as such.
This particular… whatever it is, also has a moment where the translation of it comes into play for a particular scene.
Spoilder'd For being a Major Spoiler
! In Amazon Lilly, Hancock says Luffy has the "Ambition of a King" (Ambition being "Haki")
Leaving it as "Haki", the viewer gets the notion that it's just a Random attack that means nothing, and the notion of that scene is lost on the viewer.
The Idea is that this thing, is that Luffy has "The Ambition of a King", and his Ambition is to become "Pirate King".
Translating it out gets the point of that across