How do I create a reusable block/proc/lambda in Ruby?

You can create a named Proc and pass it to the methods that take blocks:

isodd = Proc.new { |i| i % 2 == 1 }
x = [1,2,3,4]
x.select(&isodd) # returns [1,3]

The & operator converts between a Proc/lambda and a block, which is what methods like select expect.

Create a lambda and then convert to a block with the & operator:

isodd = lambda { |i| i % 2 == 1 }
[1,2,3,4].select(&isodd)

puts x.select(&method(:isodd))