[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.