How's the property of a langauge called which makes you able to write eg. a bootloader?

How's the property of a langauge called which makes you able to write eg.
a bootloader?

How is such a property of a programming language called which makes one
able to write independently of any Operating System and any
standard|runtime library? For example, one can write a boot loader in C or
in assembly for a given processor platform. How is this property[|ies]
called?