1. a person or thing that blocks.
  2. Football. a player whose assignment or special skill is blocking.
  3. Biochemistry. a substance that inhibits the physiological action of another substance, as the beta blocker propranolol that interferes with neurotransmitters in the sympathetic nervous system.


  1. a person or thing that acts as a block
  2. physiol an agent that blocks a physiological function, such as the transport of an ion across an ion channel

c.1400 of a tool, c.1600 of a person, agent noun from block (v.). U.S. football sense from 1914.

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