1. A lack of synchronization. A method of transmitting data over a network using a start bit at the beginning of a character and a stop bit at the end. The time intervak between characters may be varying lengths. In video, a signal is asynchronous when its timing differs from that of the system reference signal. From Glossary of Distance Education and Internet Terminology

2. It describes a communications strategy where start and stop bits are used to indicate the beginning and end of a character. From Linux Guide @FirstLinux