What does "pm" stand for?

drush pm is the Drush package manager.

Perhaps I'm wrong in some point but in general there's some incongruence in drupal terminology that we've hit several times in drush:

1. from drupal.org projects' view point, a project is... for example http://drupal.org/project/cck and such a project is packaged in a .tgz file. A project can contain several modules (or themes).
2. Each module, theme or profile individually (that is, something with a .info file) is also known as a project. Each of them is a project, because of the similarities (.info files, common hooks,...).
3. Moreover, modules and themes are also grouped in packages. This is the "package=" line in .info files and the fieldsets in admin/build/modules. Those packages are arbitrary, up to the developer, and has no relation with the packages in point 1.

So drush pm has some commands for packages(in the sense of drupal.org's project or .tgz) and some for projects (in the sense of individual modules and themes):
* drush dl package (.tgz)
* drush pm-releases (.tgz)
* drush pm-enable module theme
* drush pm-info module theme
....

We also use the package (module group) in pm-info command. I think a little step towards clarification could be to rename package fieldsets to 'group' in drupal and to use package in pm-info for the containing .tgz of a module, that is a more valuable info.
Package as in module groups doesn't make sense to me, but for "Core" and "Core - required" and required ones have gone away in d7, that introduces also the 'Testing' package/group.