A combination of hardware and software that allows a person to read an e-book.
The software may be built in to the hardware, or installed by the user.
Each e-reading environment has different capabilities that affect the e-reading experience.
Finally, the hardware or software may have additional capabilities, such as a web browser, wifi, and so forth.
When producing e-book files, all of these variables must be taken into account to ensure that the e-reading experience is satisfactory in all environments.