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Mail merge is a process to create personalized letters and pre-addressed envelopes or mailing labels for mass mailings from a form letter – a word processing document which contains fixed text, which will be the same in each output document, and variables, which act as placeholders that are replaced by text from the data source.
As defined in the Cambridge Dictionary Online "It's the use of a computer to produce many copies of a letter, each copy with a different name and address stored on file, or a computer program that does this."  or according to the Oxford Dictionary Online "It's the automatic addition of names and addresses from a database to letters and envelopes in order to facilitate sending mail, especially advertising, to many addresses." 
The data source is typically a spreadsheet or a database which has a field or column for each variable in the template. When the mail merge is run, the word processing system creates an output document for each row in the database, using the fixed text exactly as it appears in the template, but substituting the data variables in the template with the values from the matching columns.
Mail merge dates to early word processors on personal computers, circa 1980. WordStar was perhaps the earliest to provide this, originally via an ancillary program called MailMerge. WordPerfect also offered this capacity for CP/M and MS-DOS systems, while Microsoft Word did so later.
Mail merging requires the following steps:
- Creating a Main Document/Template.
- Creating a Data Source.
- Defining the Merge Fields in main document.
- Merging the Data with the main document.
A common usage is for creating "personalised" letters, where a template is created, with a field for "Given Name", for example. The templated letter says "Dear <Given Name>", and when executed, the mail merge creates a letter for each record in the database, so it appears the letter is more personal. It is often used for Variable Data Printing.
Another common usage is for creating address labels from a Customer Relationship Management database, or for mass emails with pertinent information in them, perhaps a username and password.
- Word processors
- Variable data printing
- Desktop publishing
- Digital printing
- Dynamic publishing
- Mass customization