a single system
whose resources are distributed 
among several nodes connected by a relatively high speed network whose
major/sole function is to support the single system.
At the hardware/physical level, there may be from little to extensive
redundancy of resources.
By increasing the degree
of physical redundancy, processing power can be added incrementally 
over a range of at least two orders of magnitude.
To the extent that hardware/physical redundancy exists, there is a single
intelligence (probably distributed) allocating resources of each class.
At the conceptual level, (user, file, input/output device
class, [F/D]TLS (DAC,MAC, etc), etc.) there is no redundancy.
E.g., there is a single user name space (possibly represented as a non-unique
user name made unique by a node qualifier), a single file name space (again, possibly non-unique file names made unique
by a node qualifier) and a single visible level of authentication.