AR stands for aspect ratio, or the ratio of the major to minor axis of an eliptical fit.
More info here (look for it under shape descriptiors).
Thank you very much for the quick response.
I have another question.
How can I get the length of a particle from this table? Does the column Major indicate the length?
@Nikhil_rao the measurements available in ImageJ are explained here:
I looked into the documentation and realized what you explained.
The elliptical fit is a model and the major and minor axis calculated are the length and width of the shape respectively?