I suggest to remove (or even blacklist) the [tag:projection] tag. The problem with this tag is that the word [projection](https://en.wikipedia.org/wiki/Projection_(mathematics)) is used in many various meanings in mathematics. 

There is projection map in geometry and in linear algebra. In linear algebra we also have projection matrix. The term projection is used in connection with Cartesian products and equivalence relations. (And also for products and quotients of various spaces or algebraic structures. For example, projection from a topological space to a quotient space.)

Keeping all these meanings under the same tag would probably make it too heterogeneous. If some of these meanings needs tag, then a tag which would be more specific can be created. (For example, the tag name could specify that the tag is intended for projections in linear algebra.)

This tag has been created and removed several times in the past.<sup>1</sup> However, I did not found previous discussions about this tag on meta.

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There already exist separate tags [tag:projection-matrices] and [tag:stereographic-projections]. And also [tag:map-projections] which seems somewhat similar and might also be problematic - I have [made a separate post](https://math.meta.stackexchange.com/questions/25694/tag-management-2017/26207#26207) about the (map-projection) tag in this thread.

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<sup>1</sup>See [this SEDE query](http://data.stackexchange.com/math/query/542457/first-10-uses-of-a-tag?tagname=projection). The first occurrence found in this way is [from 2012](https://math.stackexchange.com/posts/117418/revisions), but it only lasted for a few days. Creation of this tag has been mentioned in tagging chat room [in January 2017](http://chat.stackexchange.com/transcript/message/34924916#34924916) and again [today](http://chat.stackexchange.com/transcript/3740/2017/4/18).