1. n. [Drilling]

The set of valves, spools, and fittings connected to the top of a well to direct and control the flow of formation fluids from the well.

See: cellar,  formation fluid,  shut-in bottomhole pressure,  shut-in pressure

2. n. [Well Completions]

An assembly of valves, spools, pressure gauges and chokes fitted to the wellhead of a completed well to control production. Christmas trees are available in a wide range of sizes and configurations, such as low- or high-pressure capacity and single- or multiple-completion capacity.

See: choke,  frac stack,  frac tree,  pressure gauge,  production tree