K/T stands for Cretaceous–Tertiary, with ‘K’ rather than ‘C’ because ‘C’ had already been granted by geologists to the Carboniferous Period. Cretaceous comes from creta, the Latin for chalk, and the German for chalk is Kreide, hence the K. The ‘Tertiary’ was part of a mostly defunct system of nomenclature, which we have nevertheless indicated on the geological timescale here
