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"Chronological order" describes the series beginning with "Magician's Nephew", contrasting with "published order" which begins with "Lion, Witch..".
In a letter written in 1957 to an American boy named Laurence, Lewis wrote the following:
'I think I agree with your order {i.e. chronological} for reading the books more than with your mother's. The series was not planned beforehand as she thinks. When I wrote The Lion I did not know I was going to write any more. Then I wrote P. Caspian as a sequel and still didn't think there would be any more, and when I had done The Voyage I felt quite sure it would be the last. But I found I was wrong. So perhaps it does not matter very much in which order anyone read them. I'm not even sure that all the others were written in the same order in which they were published.'
Quoted in "Letters to Children"
There is an excellent discussion of the "two orders" on this page: http://www.aslan.demon.co.uk/narnia.htm