[fuzzfactor]

I would say 'cat /dev/sda >disk.img' is a "disk" image since it bitwise includes all sectors with partitions in order and everything on the source HDD.

You would restore a disk image to an entire HDD of at least the same size as the original, with compatible geometry when applicable.

OTOH 'cat /dev/sda1 >drive1.img' would be an image of only the first partition, making it a "drive" image of that one volume alone.

You could restore such a "drive" image to any partition of the same or larger size. Bootability would still be dependent on considerations outside that one volume.