A filter choke is an inductor in a phase-control thyristor dimmer to slow down the turn-on time of the dimmer in each half cycle. This turn-on time is known as "rise time". Higher rise times from larger chokes reduce lamp filament noise from phase-control dimmers. In high quality professional 120V thyristor dimmers typical rise times are:
350 microseconds (usec)
500 microseconds (usec)
800 microseconds (usec)--for very demanding low noise applications
The higher the rise time, the lower the efficiency of the dimmer, since larger chokes produce more voltage drop.
Typical chokes are of toroidal construction, with solid copper wire wound on a powdered iron core.
IGBT dimmers deal with rise time differently, see "Dimmer Rise Time, IGBT"
Note that rise time should never be confused with "response time"--the time the dimmer takes to turn on a load from off to its level.