In the OTC i.e., over the counter market, trades are based on contracts made directly between two parties, and not subject to the rules of an exchange. The contract terms are agreed between the parties and may be non-standard. The price will probably not be published.
Examples
Energy spot
The spot energy market allows producers of surplus energy to instantly locate available buyers for this energy, negotiate prices within milliseconds, and deliver energy in a short-term timeframe.[3] Spot markets can be either privately operated or controlled by industry organizations or government agencies. They frequently attract speculators, since spot market prices are known to the public almost as soon as deals are transacted. Examples of energy spot markets for natural gas in Europe are the Title Transfer Facility (TTF) in the Netherlands and the National Balancing Point (NBP) in the United Kingdom.