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Those portions of reservoir gas which are liquefied at the surface in field facilities or gas processing plants
  

Liquids obtained during natural gas production, including ethane, propane, butanes, and condensate.

Those hydrocarbons in natural gas that are separated from the gas as liquids through the process of absorption, condensation, adsorption, or other methods in gas processing or cycling plants. Generally such liquids consist of propane and heavier hydrocarbons and are commonly referred to as lease condensate, natural gasoline, and liquefied petroleum gases. Natural gas liquids include natural gas plant liquids (primarily ethane, propane, butane, and isobutane; see Natural Gas Plant Liquids and lease condensate (primarily pentanes produced from natural gas at lease separators and field facilities; see Lease Condensate).

Production of natural gas liquids is classified as follows:
---Contract Production. Natural gas liquids accruing to a company because of its ownership of liquids extraction facilities that it uses to extract liquids from gas belonging to others, thereby earning a portion of the resultant liquids.
---Leasehold Production. Natural gas liquids produced, extracted, and credited to a company's interest.
---Contract Reserves. Natural gas liquid reserves corresponding to the contract production defined above.
---Leasehold Reserves. Natural gas liquid reserves corresponding to leasehold production defined above.

There is no precise definition. NGLs are essentially those hydrocarbons which can be extracted in liquid form from natural gas as produced. Typically ethane, LPG, and Pentanes will be the predominant components, but there will be some heavier hydrocarbons present.