Over-The-Counter (OTC) trade is a direct off-exchange trade between two parties. OTC trades do not happen on centralised exchanges with standardised contracts and are, usually, arranged through various communications channels.
Unlike on exchanges which match buyers and sellers through one centralised order book, the OTC trade is only known to individuals involved in it.
Various crypto-currency exchanges offer OTC desks for clients with large capital. In traditional financial markets, OTC trading volumes are several times bigger than those happening on centralised exchanges.