When visiting the Google Play page of an installed app, you’re likely to see buttons labeled Open, Update, and/or Uninstall. In some cases, however, the Uninstall button might instead say Deactivate, which can seem like a weird sudden change from the normal buttons.
The explanation for this button is actually quite simple, and has to do with apps that have Device Administrator rights. Device Administration is a system that was originally intended for enterprise use in regards to security, but is also found in many normal consumer apps that in some way manipulate the device’s password lock. Examples from my own phone are Unlock With WiFi and Tasker, which both use it for being able to turn off the password lock in certain situations.
The settings for device administrators can be found in Location and Security > Device administrators in the main system settings. What’s special about these apps is that they cannot be uninstalled while they have device administrator access. As such, the Uninstall button on Google Play turns into a Deactivate button, so that you can remove that access and then uninstall.
I’ve seen people comment on apps that need this access, complaining that the app can’t be removed, and jumping to the conclusion that it’s malicious software. That’s not the case, and while the device administrator settings are a bit hidden, they’re available. So, if you’ve seen the Deactivate button, or run into an app that can’t be uninstalled, you now know why.