Basic Definition:
Reconstruction is the mathematical process through which 2D X-ray projection data (radiographs) collected using different X-ray trajectories through a sample are transformed into a 3D tomographic image, or tomogram. In a tomogram, the grayscale value of each voxel represents the X-ray linear attenuation of each point within the reconstruction volume.
A variety of reconstruction algorithms have been developed, including filtered back projection and iterative methods.