Symbol for kilo-, as in kg (kilogram) and km (kilometer).

Potassium.

Symbol for kelvin, a scale of temperature.

Abbreviation for thousand, as in a salary of $50K.

11th letter of the English alphabet, derived from the Greek kappa (K, k) and representing the voiceless velar stop. In English, it is silent before n at the beginning of a word (for example, in knee), a change accomplished, probably, in the 17th century.