Never heard of it as a generalized term for PLCs. I have heard it used to describe redundant hot backup sytems using PLCs though. In a Synchronous system, 2 CPUs are looking at I/O continuously, although only 1 of the CPUs is active. If that one goes down, the 2nd CPU is already processing the same I/O information and executing the program, so it takes over immediately (AKA Bumpless Transfer). In an Asysnchronous system, both CPUs may be running, but when the primary dies the secondary must scan the I/O status before it can establish control.
"Venditori de oleum-vipera non vigere excordis populi"