The best replacement for plugin is extension.

The best alternatives to plugin are rated as following:
★★★★★  extension
★★★★★  plug-in
★★★★  widget
★★★★  activex
★★★  module
★  gem
★  api
★  add in
★  widget wordpress
★  extension chrome
★  library
★  workflow
★  theme wordpress
★  plug in
★  receiver
★  pluginmanagement
★  module)
  continue
  volt
  functions.php

Your alternative for plugin is not listed? Suggest!
Speaking about plugin, one probably shouldn't forget about adobe flash player.

Other search terms related to plugin and their ratings are:
★★★★★ adobe flash player
★★ bukkit
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