nagware is an antipattern behavior of web sites when they pop-up a call to action that they want you to take (like install their native app), typically obscuring or even blocking what you were doing, like reading an article. registration wall and login wall are examples of nagware, as are the prompts to sign-up for an email newsletter, or install a platform specific application version of the site.
Perhaps the current poster-child for nagware, Medium, as shown in this screenshot has not one but three nagware modals that in total nearly completely obscure the actual content of the site:
1. Green OPEN IN APP prompt and modal to switch to reading in an app (when a website works just fine for reading)
2. Read here Do not track compliance
3. registration wall to read one more article