A sender and receiver operate plesiosynchronously if they operate at the same nominal clock frequency but may have a slight clock frequency mismatch, which leads to a drifting phase.
The indefinite wait and lack of external synchronization signals makes byte detection asynchronous.
Then the receiver samples at predefined intervals to determine the values of the bits in the byte; this is plesiochronous since it depends on the transmitter to transmit at roughly the same rate the receiver expects, without coordination of the rate while the bits are being transmitted.
The modern tendency in systems engineering is towards using systems that are either fundamentally asynchronous (such as Ethernet), or fundamentally synchronous (such as synchronous optical networking), and layering these where necessary, rather than using a mixture between the two in a single technology.
The term plesiochronous comes from the Greek πλησίος plesios ("near") and χρόνος chrónos ("time").