1. <programming, operating system> A formal name used in source code to refer to a variable, function, procedure, package, etc. or in an operating system to refer to a process, user, group, etc.
Each different type of entity may have a different range of valid identifiers or "name space". For example, an identifier in C is a series of one or more letters, digits and underscores that does not begin with a digit. An identifier has a type, e.g. integer variable, hash, variant and a scope, e.g. block, global.
2. <database> (id) A primary key. The column containing a table's primary key is frequently named after the table with "_id" appended, e.g. "customer_id".