Retpoline: a software construct for preventing branch-target-injection – Google Help

“Retpoline” sequences are a software construct which allow indirect branches to be isolated from speculative execution.  This may be applied to protect sensitive binaries (such as operating system or hypervisor implementations) from branch target injection attacks against their indirect branches.  

The name “retpoline” is a portmanteau of “return” and “trampoline.”  It is a trampoline construct constructed using return operations which also figuratively ensures that any associated speculative execution will “bounce” endlessly.  

(If it brings you any amusement: imagine speculative execution as an overly energetic 7-year old that we must now build a warehouse of trampolines around.)

Source: https://support.google.com/faqs/answer/7625886