A "virtual machine" or emulator that runs on PCs and can be used to run an isolated Android environment within a PC
Learn more about Virtualbox on the official website. Virtualbox is owned and distributed by Oracle, but it's GPL and free to download and use. It runs on Windows and Linux. Its main use is to run a Windows VM under Linux, or vice-versa, but it's compatible with android-x86, so it can be used to run Android too.