It is very difficult to define it - even to myself....
I do keep coming back to the light ..
drawing with light...
"Etymology[edit]
The word "photography" was created from the Greek roots φωτός (phōtos), genitive of φῶς (phōs), "light"[2] and γραφή (graphé) "representation by means of lines" or "drawing",[3] together meaning "drawing with light".[4]"
http://en.wikipedia.org/wiki/Photography