A part-select is a set of constants or constant expressions that addresses several contiguous bits of a Verilog HDL vector net or register. The constants or constant expressions of a part-select are enclosed in brackets; the MSB of the range is shown first; the LSB is shown last. The start and end of the part-select are separated by a colon. Part-selects are sometimes called "slices."
See "Section 4.2.1: Vector bit-select and part-select addressing" in the IEEE Std 1364-2001 IEEE Standard Hardware Description Language manual for more information.