A steam trap is a device used to discharge condensate and non condensable gases with a negligible consumption or loss of live steam.
Ainsi defines steam traps the following way
steam trap - Self contained valve which automatically drains the condensate from a steam containing enclosure while remaining tight to live steam, or if necessary, allowing steam to flow at a controlled or adjusted rate. Most steam traps will also pass non-condensible gases while remaining tight to live steam.
ANSI/FCI 69-1-1989
