I only know that noitaminA is "Animation" written backwards. It sometimes appear at the beginning of episodes of anime.
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As @LoganM points out in a comment, noitaminA is a programming block on Fuji TV that initially went from 24:45-25:15 on Thursday nights (i.e. 12:45a-1:15a on Friday) and was later lengthened to 24:45-25:45.
Old noitaminA shows tended to have one major commonality - they targeted a different audience than most other late-night anime. Some good examples of this are:
While the shows aired by noitaminA do not have any single unifying feature, "not being like a typical late-night anime show" is an interesting enough feature to merit a tag or genre on sites like Anime-Planet, I suppose. One other unusual feature of noitaminA shows is that they typically (though not always) last 11 or 22 episodes, rather than 12-13 or 24-26.
Some argue that the past couple years of noitaminA have been characterized by very conventional anime, rather than the mold-breakers of noitaminA's early days, thus diluting the value of noitaminA as a marker of quality. Examples: