what are "only ichi" photographs??

One of the ongoing challenges is for "only ichi" photos.  But there's nothing in the rules that explains what this is.  Am I hopelessly behind the times, and Ichi is some big photography movement that everyone else immediately recognizes?  I tried Google, but had no luck, which makes me believe that (a) I must be missing something, and (b) I am not the only one?

On all the prior DP challenges that ask for some particular style, the host gives a definition, a link to Wikipedia, etc.  But nothing here.

Can anyone clue me in?  Thanks . . . I hate feeling culturally ignorant.  (I learned just this month what Gangnam Style means, so that gives you all an idea of my 'hipness' level.)

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http://www.pbase.com/santamonica
http://www.strataphotography.com