Raised to stop the iteration, in particular by Enumerator#next. It is rescued by Kernel#loop.

loop do
  puts "Hello"
  raise StopIteration
  puts "World"
end
puts "Done!"

produces:

Hello
Done!
Instance Methods

Returns the return value of the iterator.

o = Object.new
def o.each
  yield 1
  yield 2
  yield 3
  100
end

e = o.to_enum

puts e.next                   #=> 1
puts e.next                   #=> 2
puts e.next                   #=> 3

begin
  e.next
rescue StopIteration => ex
  puts ex.result              #=> 100
end