This is a dimension in reciprocal space, i.e. nm^-1. I would suggest to search for some images using google since then you perhaps understand it immediately.

Hi Syam, here are the sheet of my lecture regarding your question and the windows of ImageJ and DigitalMicrograph to define the pixel size:

(but the unit of a diffraction pattern or a FFT is of course 1/nm!!)

http://imagej.nih.gov/ij/

http://www.gatan.com/products/tem-analysis/gatan-microscopy-suite-software

Thank you very much for yours suggestions Gert Nolze sir and Vinod kumar sir. I am thankful to Volker Klemm sir for providing the image j and gatan software links.