General computing term. Also known as native code.
Wikipedia: "...Unmanaged code refers to programs written in C, C++, and other languages that do not need a [common language] runtime [virtual machine] to execute."
Stackoverflow: "Native code is the code whose memory is not 'managed', as in, memory isn't freed for you (C++' delete and C's free, for instance), no reference counting, no garbage collection."
See also: managed code.