What are the giant walkers used by the Empire on Hoth called?

The story behind the answer

On Hoth, the Empire deploys massive, four-legged All Terrain Armored Transports for heavy assault and troop/cargo delivery; their towering height and thick armor shrug off standard blaster fire. The Battle of Hoth in The Empire Strikes Back famously shows Rebel snowspeeders tripping them with tow cables, exploiting their limited maneuverability.

Don’t confuse them with the AT-ST (a smaller, two-legged “chicken walker” scout), the AT-PT (a single-seat light patrol walker), or the AT-TE (a six-legged Clone Wars–era Republic tank, not Imperial). Fun fact: these walkers were brought to life with groundbreaking stop-motion effects by Phil Tippett’s team at ILM.