Disk imaging is the process of making a bit-by-bit copy of a disk. Imaging (in more general terms) can apply to anything that can be considered as a bit-stream, e.g. a physical or logical volumes, network streams, etc.
The most straight-forward disk imaging method is reading a disk from start to end and writing the data to a Forensics image format. This can be a time consuming process especially for disks with a large capacity.