A silo is a tall,
cylindrical building. Initially, as
Webster 1913 says, it was used to pack
grains for
storage. These are referred to as
grain silos.
The twentieth century saw a new type of silo, the
missile silo. These enclosures were used to raise
missiles
perpendicular to the ground and launch them, usually for
impact on
ground owned by someone disliked by the silo's builder.
Ironically, the original type of silo has more interesting
explosion lore bound up with it. Imagine a silo full of
flour. Now, flour is
combustible, but if you put a match to a pile of flour not much happens, because only a little bit of the flour's area is exposed to the
air, where it can react with the
oxygen. But when our silo is supposedly
empty, there is actually a good deal of flour floating in the air, like
dust. Now, it's all exposed at once and can all
combust at once. A stray
spark can cause a massive
explosion. Like much of
applied physics, it is simultaneously
amazing and
not funny at all.
For more information, see
blow up a building with a sack of flour and two rounds of ammunition -- a
node which I neither authored nor endorse, but which does illustrate the
principle nicely.