Numerical Aperture (NA) can be defined as the light gathering ability of a microscope and is a dimensionless number that gives the range of angles at which a system can emit light. Its exact meaning varies in different areas of optics and in terms of optical system it can be defined as :
NA = n * sin theta
To read more about Numerical Aperture, follow this link : http://en.wikipedia.org/wiki/Numerical_aperture