Asynchronous transmission is the transmission of data in which each character is a self-contained unit with its own start and stop bits and an uneven interval between them.
Asynchronous transmission is also referred to as start/stop transmission.
Techopedia explains Asynchronous Transmission
Asynchronous transmission uses start and stop bits to signify the beginning and ending bits. The additional one at the start and end of a transmission alerts the receiver to the occurrence of the first character and last character. The asynchronous transmission method is deployed when data is sent as packets as opposed to in a solid stream. The start and stop bits have opposite polarity, allowing the receiver to understand when the second packet of information has been sent.
The two main characteristics specific to asynchronous communication are:
Every character is preceded by a start bit and followed by one or more stop bits
Spaces between characters are common