The best replacement for extension is flexion.

The best alternatives to extension are rated as following:
★★★★★  flexion
★★★★  extension
★★  conservatory
★★  expansion
★★  renewal
★  plugin
★  extention
★  intension
★  line extension
★  brand extension
★  app
★  loft conversion
★  add on
★  load graph
★  squat
★  orangery
  variation
  hyperextension
  intention
  weave
  customization
  category
  no extension
  back flexion
  contract renewal
  leg press
  reenlistment
  leg curl
  inheritance
  bhiwadi
  new brand
  category extension
  torsion garage door springs
  torsion
  new build
  enrichment
  lumbar flexion
  abduction
  amendment
  hip flexion

Your alternative for extension is not listed? Suggest!
Speaking about extension, one probably shouldn't forget about torque wrench.

Other search terms related to extension and their ratings are:
★★★★★ torque wrench
★★ payment
★★ code
★ penalty
You have in mind a companion for extension and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of extension in all years. Click on graph for more.
extension
flexion
extension