It is one that can run purely without electronics, no ECM (Electronic Control Module).
The biggest danger from a massive solar storm or EMP (electromagnetic pulse) is to anything "on the grid" (hooked to power lines, which ultimately are giant antennas). Ever since the advent of the telegraph, solar flares have been playing havoc with long distance wiring - frying insulators, etc. Automotive electronics are far less vulnerable, but not completely so.
"Pure" diesels don't even need spark plugs, much less electronic timing and fuel injection, to operate.