The null character (also null terminator or null byte), abbreviated NUL or NULL, is a control character with the value zero. It is present in many , including ISO/IEC 646 (or ASCII), the C0 control code, the Universal Coded Character Set (or Unicode), and EBCDIC. It is available in nearly all mainstream programming languages."A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string literal." — ANSI/ISO 9899:1990 (the ANSI C standard), section 5.2.1
The original meaning of this character was like NOP—when sent to a computer printer or a terminal, it does nothing (some terminals, however, incorrectly display it as space). When electromechanical were used as computer output devices, one or more null characters were sent at the end of each printed line to allow time for the mechanism to return to the first printing position on the next line. On punched tape, the character is represented with no holes at all, so a new unpunched tape is initially filled with null characters, and often text could be "inserted" at a reserved space of null characters by punching the new characters into the tape over the nulls.
Today the character has much more significance in C and its derivatives and in many data formats, where it serves as a reserved character used to signify the end of a string,"A string is a contiguous sequence of characters terminated by and including the first null character" — ANSI/ISO 9899:1990 (the ANSI C standard), section 7.1.1 often called a null-terminated string. This allows the string to be any length with only the overhead of one byte; the alternative of storing a count requires either a string length limit of 255 or an overhead of more than one byte (there are other advantages/disadvantages described under null-terminated string).
The ability to represent a null character does not always mean the resulting string will be correctly interpreted, as many programs will consider the null to be the end of the string. Thus the ability to type it (in case of unchecked user input) creates a vulnerability known as null byte injection and can lead to security exploits. Null Byte Injection WASC Threat Classification Null Byte Attack section.
In caret notation the null character is ^@. On some keyboards, one can enter a null character by holding down and pressing (which usually requires also holding and pressing another key such as or ).
In documentation, the null character is sometimes represented as a single-em-width symbol containing the letters "NUL". In Unicode block, there is a character with a corresponding glyph for visual representation of the null character, "symbol for null", U+2400 (␀)—not to be confused with the actual null character, U+0000.