1. Asynchronous transmission in which a start pulse and a stop pulse are used for each symbol.
2. Signaling in which each group of code elements corresponding to an alphanumeric character is (a) preceded by a start signal that serves to prepare the receiving mechanism for the reception and registration of a character and (b) followed by a stop signal that serves to bring the receiving mechanism to rest in preparation for the reception of the next character.
Source: from Federal Standard 1037C and from MIL-STD-188