It is the big fatty hump on fish's head.. find them on mature red devils, flower horns, frontosa etc.. it is mostly made up of fatty deposits.. and more prominant on male fish...i would show you a pic except while MFK is upgrading to the next level.. the attachments are not working unless you use a different program.. but you can see the pics if you look at the cichlids...
Kok is a primarily FH related word which im sure stems from malaysia or thailand, whereas nuchal hump is the preffered scientific term for a lump on the head of cichlids mostly males to indicate dominance, potency and of course to get a mate. some species naturally have larger nuchal humps then others, but because flowerhons have been specifically bred to show these characteristics favourable in asian countrys the term "Kok" has been applied.
on some cichlids the nuchal hump it is slight
here is a Midas with a pronounced one
And here is a Flowerhorn with an oversized KOK. this would be virtually impossible in a wild Midas or red devil
because it is a trait that is specificaly bred into these fish.
some flowerhorns like this Kamfamalau take on the look of a fully mature large american cichlid at small sizes.
this is obviously the prefered trend when breeding these fish.