The best replacement for subdomain is subdirectory.

The best alternatives to subdomain are rated as following:
★★★★★  subdirectory
★★★  domain
★★★  folder
★★  directory
★★  new domain
★★  subfolder
★★  subdirectory seo
★★  addon domain
★  subfolder seo
★  subdirectory wordpress
★  hostname
★  domain seo
★  child domain
★  new domain seo
★  cname
★  alias
  top level domain
  virtual directory
  add on domain
  root domain
  subdirectory wordpress multisite
  separate domain
  sub directory

Your alternative for subdomain is not listed? Suggest!
Speaking about subdomain, one probably shouldn't forget about www.

Other search terms related to subdomain and their ratings are:

★★★★★ www
★★ cloudflare
★★ no ads
www prefix
You have in mind a companion for subdomain and it is not yet here? Propose!

Google Trends

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