a person or thing that blocks.
Football. a player whose assignment or special skill is blocking.
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.

1200–50; Middle English; see block, -er1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
block (blɒk)
1.  a large solid piece of wood, stone, or other material with flat rectangular sides, as for use in building
2.  any large solid piece of wood, stone, etc, usually having at least one face fairly flat
3.  such a piece on which particular tasks may be done, as chopping, cutting, or beheading
4.  Also called: building block one of a set of wooden or plastic cubes as a child's toy
5.  a form on which things are shaped or displayed: a wig block
6.  slang a person's head (esp in the phrase knock someone's block off)
7.  slang (Austral), (NZ) do one's block to become angry
8.  a dull, unemotional, or hardhearted person
9.  a large building of offices, flats, etc
10.  a.  a group of buildings in a city bounded by intersecting streets on each side
 b.  the area or distance between such intersecting streets
11.  (Austral), (NZ) an area of land for a house, farm, etc
12.  (Austral), (NZ) a log, usually a willow, fastened to a timber base and used in a wood-chopping competition
13.  an area of land, esp one to be divided for building or settling
14.  See cylinder block
15.  a.  a piece of wood, metal, or other material having an engraved, cast, or carved design in relief, used either for printing or for stamping book covers, etc
 b.  (Brit) a letterpress printing plate, esp one mounted type-high on wood or metal
16.  See also block and tackle a casing housing one or more freely rotating pulleys
17.  chiefly (US), (Canadian) on the block up for auction
18.  the act of obstructing or condition of being obstructed, as in sports
19.  an obstruction or hindrance
20.  pathol
 a.  interference in the normal physiological functioning of an organ or part
 b.  See heart block
 c.  See nerve block
21.  psychol a short interruption of perceptual or thought processes
22.  obstruction of an opponent in a sport
23.  a.  a section or quantity, as of tickets or shares, handled or considered as a single unit
 b.  (as modifier): a block booking; block voting
24.  a.  a stretch of railway in which only one train may travel at a time
 b.  (as modifier): a block signal
25.  Compare strip an unseparated group of four or more postage stamps
26.  a pad of paper
27.  computing a group of words treated as a unit of data on a tape, disk, etc
28.  athletics short for starting block
29.  cricket a mark made near the popping crease by a batsman to indicate his position in relation to the wicket
30.  informal a chip off the old block a person who resembles one of his or her parents in behaviour
31.  to shape or form (something) into a block
32.  to fit with or mount on a block
33.  to shape by use of a block: to block a hat
34.  (often foll by up) to obstruct (a passage, channel, etc) or prevent or impede the motion or flow of (something or someone) by introducing an obstacle: to block the traffic; to block up a pipe
35.  to impede, retard, or prevent (an action, procedure, etc)
36.  to stamp (a title, design, etc) on (a book cover, etc) by means of a block (see sense 12), esp using gold leaf or other foil
37.  (esp of a government or central bank) to limit the use or conversion of assets or currency
38.  (also intr) sport to obstruct or impede movement by (an opponent)
39.  (intr) to suffer a psychological block
40.  to interrupt a physiological function, as by use of an anaesthetic
41.  (also intr) cricket to play (a ball) defensively
[C14: from Old French bloc, from Dutch blok; related to Old High German bloh]

blocker (ˈblɒkə)
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

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
Propranolol is a beta-blocker, so it blocks the action of adrenaline.
It is also being evaluated in combination with selective alpha-blocker drugs.
Excellent rebounder and shot-blocker but his offensive game is lacking.
Propranolol, a beta blocker drug, has no beneficial effect on the symptoms of
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