safety valve

Short definition from Wiktionary

Noun

1. 1
   A valve set to open at a pressure below that at which a container holding a gas, vapour, etc. (such as a boiler or pressure cooker), would burst, thus reducing the pressure; a relief valve.
2. 2
   A valve set to close a container holding a gas or vapour to prevent excessive loss of pressure.
3. 3
   Any mechanism offering relief from physical or emotional pressure or tension.
4. 4
   Any mechanism offering relief from physical or emotional pressure or tension: A United States law, codified at 18 United States Code §3553(f), authorizing a judge to disregard mandatory minimum sentences for some criminals with few or no prior offenses.
5. 5
   Alternative form of safety valve.

Verb

1. 1
   Alternative form of safety-valve.
2. 2
   Sometimes followed by off: To use a relief valve on (a container holding a gas or vapour) to relieve excessive pressure which has built up inside.
3. 3
   Sometimes followed by off: To install a relief valve on (such a container) for that purpose.
4. 4
   Sometimes followed by off: To use some mechanism to relieve (physical or emotional pressure or tension).