Identifier

A 16-bit field in the IP packet header that is used in conjunction with the Flags and Fragment Offset fields for packet fragmentation. Packets must be fragmented if the original length exceeds the Maximum Transmission Unit (MTU) of a data link that they travel on. If they are fragmented, the router marks each fragment with the same number in the Identifier field so that the receiving device is able to reassemble the packets.

A type of token extracted during the lexical analysis phase of compiling a computer program. Most programming languages¹ define an identifier as a letter² followed by any number of letters or digits. However, most languages³ reserve certain of these character sequences as keywords, not allowing programmers to make identifiers from them.

Programmers create identifiers to name things in their programs:

• macros
• constants
• types
• variables (aka objects)
• routines (aka procedures, functions, and methods)

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¹There is at least one language that confuses identifiers and string literals, and infers which is which only from context.
²The definition of a "letter" varies between computer languages. Some allow only upper case letters, but most allow lower case letters and/or underscores.
³Again, some languages recognize keywords only where they are meaningful as keywords, and allow them to be used as identifers! There is the opposite example of C++ which reserves any sequence beginning with an underscore followed by a capital letter or another underscore for compiler and library implementors.