The term AM window refers to a range of amateur radio frequencies historically used by AM aficionados. The most obvious example is the range around 3875-3890 KHz, with activity centered around 3885 KHz. Here you will hear many very loud, professional sounding amateur stations, and a group of acerbic personalities. Timtron is one such personality; Uncle Ed is another.
Sometimes, music and other types of material are played by unidentified parties in the AM window. It may not be appropriate to call this "pirate" activity, as the operators are almost certainly amateur radio operators. Often times, this type of activity is to harass or amuse other operators.
Also, pirate radio programs are sometimes heard in the AM window. Beerus Maximus once heard Radio Clandestine on 3885 KHz, suggesting a link between AM enthusiasts and at least some pirate radio stations.
One AM window personality, Jamie KF2VM, was busted for operating Hope Radio International in the early 1990's.