What is 'K' toner?
The letter 'K' is generally used to describe BLACK toner or ink
OKI use the letter 'K' when referring to the black toner and the black image drum.
The toner cartridges and image drums are represented by the four colours in the CMYK colour model:: Cyan, Magenta, Yellow and BlacK.
There is no definitive reason why the letter 'K' is used but some possible suggestions are:
The 'K' in CMYK stands for 'Key' since in four-colour printing the Cyan, Magenta and Yellow printing plates are carefully keyed or aligned with the key of the Black key plate.
The letter 'K' is used because it's the last letter of the word "Black" and is not occupied by any other colour. i.e. the letter "B" could not be used as it's an abbreviation used for the colour blue in the RGB colour model.