A software function which provides the ability to create multiple (and potentially large numbers of) documents from a single template form and a structured data source.
Mail merge is a software function which provides the ability to create multiple (and potentially large numbers of) documents from a single template form and a structured data source. Often these documents are letters and/or labels to be sent to a number of recipients (hence the term).
It is expected that questions about mail merging relate to programming in some way. You should tag your question with the programming language or technology you are using to perform the mail merge.