Mainly it means lower-than-OS, such as firmware, hypervisor layers, hardware backdoors/leaks, etc. Things that are guaranteed to be invisible to the guest layer

Mainly it means lower-than-OS, such as firmware, hypervisor layers, hardware backdoors/leaks, etc. Things that are guaranteed to be invisible to the guest layers. The other pre-OS-malware meaning is HDs and flash devices shipped with viruses on them.