What is middleware when referenced in the context of Ruby on Rails?

What is middleware when referenced in the context of Ruby on Rails?

ruby-on-rails ruby rack-middleware

ruby

"use" keyword/word in Ruby/Rails/Rack code

ruby-on-rails ruby rack rack-middleware

ruby

How to use Middleware from an Engine in an Engine

ruby-on-rails ruby middleware rails-engines rack-middleware

ruby

RoR - Which is preferred - Rack Middleware or Active Controller Filters?

ruby-on-rails ruby ruby-on-rails-4 actioncontroller rack-middleware

ruby