The term postback has three meanings, depending on the context, one in relation to eCommerce as a web service, another in relation to web development, and one in relation to open-source software

Open-source software

Open-source software is computer software that is available in source code form: the source code and certain other rights normally reserved for copyright holders are provided under a software license that permits users to study, change, improve and at times also to distribute the software.Open...

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In eCommerce

In the context of eCommerce, the term is used to describe a sales transaction notification from payment processors to the merchants affiliate system site. Specifically a web service written for an affiliate sales tracking software system for third party merchant system to send or "POST" the data to. The term "Postback" is used here to describe what the payment processor does with the transaction receipt, they "Post Back" to the merchants affiliate program, notifying them of a successful transaction, where they can then credit affiliates with their earnings.

In web development

In the context of web development, a postback is an HTTP POST

POST (HTTP)

In computing, POST is one of many request methods supported by the HTTP protocol used by the World Wide Web. The POST request method is used when the client needs to send data to the server as part of the request, such as when uploading a file or submitting a completed form.In contrast to the GET...

 to the same page that the form is on. In other words, the contents of the form are POSTed back to the same URL as the form. Postbacks are most commonly discussed in relation to ASP or ASP.NET

ASP.NET

ASP.NET is a Web application framework developed and marketed by Microsoft to allow programmers to build dynamic Web sites, Web applications and Web services. It was first released in January 2002 with version 1.0 of the .NET Framework, and is the successor to Microsoft's Active Server Pages ...

. In ASP, a form and its POST action have to be created as two separate pages, resulting in the need for an intermediate page and a redirect if one simply wants to perform a postback. This problem was addressed in ASP.NET with the __doPostBack function and an application model that allows a page to perform validation and processing on its own form data.

Postbacks are commonly seen in edit forms, where the user updates information in a form, hits save or submit, and the page is refreshed with a new form populated with the recently updated information.

In open source

In the context of open source software, the term "postback" is often used to describe making your changes available in source code form. Many licenses require a postback to the original author or to the general public. Most such licenses require that this modified code be licensed under the terms of the original license.