[saurik]

That's generally called a "disk image", not a "drive image", which is a term I had never heard before in my over 30 years of using computers and seems to almost exclusively be used with relation to some proprietary software program that happens to have that name.

[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.