[kmm01]

The C standard defines them very differently though:

  undefined behavior
    behavior, upon use of a nonportable or erroneous program
    construct or of erroneous data, for which this International
    Standard imposes no requirements

  unspecified behavior
    use of an unspecified value, or other behavior where this
    International Standard provides two or more possibilities
    and imposes no further requirements on which is chosen in
    any instance

Implementations need not but may obviously assume that undefined behavior does not happen. Assume that however the program behaves if undefined behavior is invoked is how the compiler chose to implement that case.

[marssaxman]

"Nonportable" is a significant element of this definition. A programmer who intends to compile their C program for one particular processor family might reasonably expect to write code which makes use of the very-much-defined behavior found on that architecture: integer overflow, for example. A C compiler which does the naively obvious thing in this situation would be a useful tool, and many C compilers in the past used to behave this way. Modern C compilers which assume that the programmer will never intentionally write non-portable code are.... less helpful.