日本語 Japanese Ask Questions Thread 4

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Japanese Ask Questions Thread2
http://dis.4chan.org/read/lang/1206158123/1-40

Japanese Ask Questions Thread3
http://dis.4chan.org/read/lang/1267485093/1-40

It's been a very long time coming but I finally understand というのは/とは but I still have one lingering hang up about the pattern it's typically used with. From ALC 500 daily expressions:

教育ママとは自分の子供の教育に熱心な母親のことです。
水蒸気というのは気体の状態に変わった水のことである。

I don't understand why the final nouns are nominalized with こと. I understand the 名詞のこと = "about" construction but I get the sense this is a completely different 名詞のこと. It seems redundant somehow. Thanks for your time : )