cl is the written short form of centilitre or centilitres, a small measure of liquid: one hundredth of a litre, which is ten millilitres. You see it on some drink labels, especially wine and spirits with European numbering — a usual wine bottle is 75 cl. It is for the page, not for speech: in a sentence people say the full unit, or they just read the number. In Taiwan shops you are much more likely to see ml. Do not mix the small letters cl with Cl in science, which stands for chlorine.

釐升（cL；等於 10 毫升。台灣瓶身多標 ml）