The Collaborative International Dictionary of English v.0.48:

Block \Block\ (bl[o^]k), v. t. [imp. & p. p. Blocked
   (bl[o^]kt); p. pr. & vb. n. Blocking.] [Cf. F. bloquer, fr.
   bloc block. See Block, n.]
   1. To obstruct so as to prevent passage or progress; to
      prevent passage from, through, or into, by obstructing the
      way; -- used both of persons and things; -- often followed
      by up; as, to block up a road or harbor; to block an
      entrance.
      [1913 Webster]

            With moles . . . would block the port. --Rowe.
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            A city . . . besieged and blocked about. --Milton.
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   2. To secure or support by means of blocks; to secure, as two
      boards at their angles of intersection, by pieces of wood
      glued to each.
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   3. To shape on, or stamp with, a block; as, to block a hat.
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   4. to cause (any activity) to halt by creating an
      obstruction; as, to block a nerve impulse; to block a
      biochemical reaction with a drug.
      [PJC]

   To block out, to begin to reduce to shape; to mark out
      roughly; to lay out; to outline; as, to block out a plan.
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The Collaborative International Dictionary of English v.0.48:

Blocking \Block"ing\, n.
   1. The act of obstructing, supporting, shaping, or stamping
      with a block or blocks.
      [1913 Webster]

   2. Blocks used to support (a building, etc.) temporarily.
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WordNet (r) 3.0 (2006):

blocking
    n 1: the act of obstructing or deflecting someone's movements
         [syn: blocking, block]