blocker

noun

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.

noun

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