The Free On-line Dictionary of Computing (18 March 2015):
The range of addresses which
a processor or process can access, or at which a device can
be accessed. The term may refer to either physical address
or virtual address.
The size of a processor's address space depends on the width
of the processor's address bus and address registers.
Each device, such as a memory integrated circuit, will have
its own local address space which starts at zero. This will
be mapped to a range of addresses which starts at some base
address in the processor's address space.
Similarly, each process will have its own address space,
which may be all or a part of the processor's address space.
In a multitasking system this may depend on where in memory
the process happens to have been loaded. For a process to be
able to run at any address it must consist of
position-independent code. Alternatively, each process may
see the same local address space, with the memory management
unit mapping this to the process's own part of the
processor's address space.