Meaning of fly block

fly' block"

Pronunciation: [key]
— Mach., Naut. Mach.,Naut.
  1. (in a Spanish burton or the like) a block, supported by a runner, through which the hauling part of the fall is rove.
  2. any block that shifts with the movement of its tackle.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.