What is the definition for "milestone" and how is it supposed to use?
Not listed on https://www.mediawiki.org/wiki/Team_Practices_Group/Glossary

See https://www.mediawiki.org/wiki/Phabricator/Creating_and_renaming_projects#Guidelines for required data in project creation requests.

I have added my intended definition of Milestone to the TPG glossary. Teams which are working with Phlogiston will use the Milestone tag to designate tasks representing significant goals, such as quarterly goals or immediate components of quarterly goals. If these tasks have multiple "descendents" -- not really descendents, since the Blocked By relationship in Phabricator is not a parent-child relationship, but it is often used in that way and we will use it in that way for this purpose -- these descendents can all be accounted to the Milestone task for purposes of velocity tracking and forecasting. This would provide a new answer to the question How should TPG associate goals with tasks?. I would like to get @DarTar's feedback on whether this would be useful or conflicting for his Dao of DarTar method.

Once upon a time we had a Report project in https://phabricator.wikimedia.org/project/profile/1082/ which got partially superseded by Roadmap in https://phabricator.wikimedia.org/project/profile/1109/ which didn't work (and, for completeness, partially superseded by Notice etc used for weekly Tech News).
For quarterly goals we had 201516-Q1 in https://phabricator.wikimedia.org/project/profile/1447/ which was never used.

Hence my curiosity why the proposed approach would work better and how to make sure that people will actively and consistently use this tag.

I'm intending to use it actively in piloting Milestone-based Phlogiston forecasting. If the pilot doesn't produce results, it could be archived with the other tags, I guess. Is there a particular downside relative to the potential benefit?

Is this task somehow related to T120175: Use parent tasks as categories? Asking as this task mentions parent tasks, but not categories.

Yes. It goes in parallel with T115936: Provide Blocked By relationships in public dump: having parent-child relationship in the dump is useful, but you still need some way to figure out _which_ parents are of interest. Milestone tag solves that problem. Epic is close, but not the same thing because not all Epics are Milestones.

Could you provide a good description for this tag, please? See https://www.mediawiki.org/wiki/Phabricator/Creating_and_renaming_projects#New_projects

This tag designates a task and its descendents for reporting within Phlogiston. If you are reporting with Phlogiston, use it to define significant stopping points within planned work (e.g., a quarterly goal or portion thereof). See the Glossary for a more complete definition. See https://www.mediawiki.org/wiki/Phlogiston for usage within Phlogiston.