speedometer n.
A pattern of lights displayed on a linear set of LEDs (today)
or nixie tubes (yesterday, on ancient mainframes). The pattern is
shifted left every N times the operating system goes through its
main loop. A swiftly moving
pattern indicates that the system is mostly idle; the speedometer
slows down as the system becomes overloaded. The speedometer on
Sun Microsystems hardware bounces back and forth like the eyes on
one of the Cylons from the wretched "Battlestar Galactica" TV
series.
Historical note: One computer, the GE 600 (later Honeywell
6000) actually had an analog speedometer on the front
panel, calibrated in instructions executed per second.