What is Over-the-counter?

Legal Definition
Over-the-counter (OTC) or off-exchange trading is done directly between two parties, without the supervision of an exchange. It is contrasted with exchange trading, which occurs via exchanges. A stock exchange has the benefit of facilitating liquidity, mitigates all credit risk concerning the default of one party in the transaction, provides transparency, and maintains the current market price. In an OTC trade, the price is not necessarily published for the public.

OTC trading, as well as exchange trading, occurs with commodities, financial instruments (including stocks), and derivatives of such products. Products traded on the exchange must be well standardized. This means that exchanged deliverables match a narrow range of quantity, quality, and identity which is defined by the exchange and identical to all transactions of that product. This is necessary for there to be transparency in trading. The OTC market does not have this limitation. They may agree on an unusual quantity, for example. In OTC market contracts are bilateral (i.e. contract between only two parties), each party could have credit risk concerns with respect to the other party. OTC derivative market is significant in some asset classes: interest rate, foreign exchange, stocks, and commodities.

In 2008 approximately 16 percent of all U.S. stock trades were "off-exchange trading"; by April 2014 that number increased to about forty percent. Although the notional amount outstanding of OTC derivatives in late 2012 had declined 3.3% over the previous year, the volume of cleared transactions at the end of 2012 totalled US$346.4 trillion. "The Bank for International Settlements statistics on OTC derivatives markets showed that notional amounts outstanding totalled $693 trillion at the end of June 2013... The gross market value of OTC derivatives – that is, the cost of replacing all outstanding contracts at current market prices – declined between end-2012 and end-June 2013, from $25 trillion to $20 trillion."
-- Wikipedia
Legal Definition
1. Retailing: A drug or medication that does not require a doctor's prescription. 2. Securities: Unlisted US securities (usually unlisted because the issuer does not meet the exchange's requirements).
Legal Definition
Any transaction that doesnt take place in person.