Should the "install" button on admin/modules page say "enable"?
My understanding is that not all modules that are available in Drupal are installed. I am speaking for Drupal 7 as I am not sure about Drupal 8.

There are three phases:


Enabled: The code is downloaded to your server and the hooks in .module are being called.
Disabled: The code is downloaded to your server, but the hooks are being ignored.
Not-Installed: The code is downloaded to your server, but Drupal hasn't run the .install file for the module and/or hasn't been made aware to look for the hooks. 


Also, it's worth noting that if you disable a module, all your data and configurations remain, while uninstalling a module invokes the uninstall hook and wipes both.

So as you can see, the label is relevant to whether or not the module needs to be installed or enabled. You can go from not-installed directly to enabled, but you must go from enabled to disabled to not installed especially before removing the module by deleting it's files.

https://www.drupal.org/node/1176662