A shed is a unit used in particle physics. It is used to express the area of a section of a particle from which other particles are scattered. So it is very very small.
The barn is another very small unit used for the same thing, though the shed is much much smaller.
I think the unit "barn" was named from that old saying "Couldn't hit the broad side of a barn." then the "shed" came when they needed something even smaller.
