In telegraphy, one of the two significant conditions of encoding. Note 1: The complementary significant condition is called a "yyspace. " Note 2: In modern digital communications, the two corresponding significant conditions of encoding are called "1" and "0. " Synonyms marking pulse, marking signal. 2. A symbol or symbols that indicate the beginning or the end of a field, of a word, or of a data item in a file, record, or block.