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A basic formal definition would be a digital product is created inside a digital format to be used about computers and other electronic devices and is also not tangible otherwise.

I do not really like to offer just official definitions though as they often offer very little. So from your marketing point of view:

A digital product is a monetisation of your time, information and effort. It is how “new money” is made and many individuals who do it correctly are becoming very rich from it.
A digital system is superior to a physical product in many ways. You only create that once (you can offer updates though) and it can be purcahased by hundreds, thousands or even numerous persons without no reduce. This ensures a certain a higher level profitability.
You can create an entire online business/blog around providing digital products.
They can be e-books, audio, graphics, whitepapers, computer software, photographs and much more.