Page "Plesiochronous system" Paragraph 2

The asynchronous serial communication protocol is asynchronous on the byte level, but plesiochronous on the bit level.

The receiver detects the start of a byte by detecting a transition that may occur at a random time after the preceding byte.

The indefinite wait and lack of external synchronizaton 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.