spl

spl /S-P-L/  [abbrev, from Set Priority Level] The way
   traditional UNIX kernels implement mutual exclusion by running code
   at high interrupt levels.  Used in jargon to describe the act of
   tuning in or tuning out ordinary communication.  Classically, spl
   levels run from 1 to 7; "Fred's at spl 6 today" would mean that
   he is very hard to interrupt.  "Wait till I finish this; I'll spl
   down then."  See also {interrupts locked out}.