Liquid solutions (referred to as mud) conveyed at high pressure through the high-pressure mud piping system, mud standpipe, rotary hose, rotary swivel stem, drill string, and drill bit to accommodate the drilling process.
NOTE: For the purpose of this specification, drilling liquids do not include fluids containing pressurized air or gasses of any kind.
Source: API SPEC 7K, Drilling and Well Servicing Equipment Upstream Segment, Fifth Edition, June 2010 (August 2010). Global Standards
Source:API SPECIFICATION 7K, Drilling and Well Servicing Equipment, Sixth Edition, December 2015. Global Standards