OTC is the short form of over-the-counter. The picture is: you stand at a counter and buy something yourself, with no special permission in between. The meaning many learners meet first is medicine. An OTC medicine is one you can buy in a pharmacy without a doctor’s prescription — cold tablets, painkillers, some creams. The opposite is a prescription drug. The other big meaning is money and markets. OTC trading is buying and selling that does not go through a main stock exchange; the deal happens “over the counter” between two parties, or on a smaller market. Though actually, in Taiwan this second meaning is very loud: people say OTC when they talk about the 櫃買 / 店頭 market, and a student here may think of stocks before they think of a chemist. It is an abbreviation, fairly neutral in tone — medicine boxes, finance news, job talk. Always let the context choose: pills, or prices.