The best replacement for async is sync.

The best alternatives to async are rated as following:
★★★★★  sync
★★★★★  q
★★★  defer
★★★  thread
★★  sync ajax
★★  sync c#
★★  sync nfs
★★  defer javascript
★★  step
★  sync javascript
★  backgroundworker
★  underscore
★  sync overclocking
★  bluebird
★  task c#
★  sync ssd
★  task
★  tpl
★  thread c#
  sync java
  dispatch_sync

Your alternative for async is not listed? Suggest!
Speaking about async, one probably shouldn't forget about task c#.

Other search terms related to async and their ratings are:
★★★★★ task c#
★ await
You have in mind a companion for async and it is not yet here? Propose!



Google Trends

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async
sync
q