Lit. shape, form
Katachi means maintaining a proper Body position at all times.
Proper Body position translates into the model Tachiai as well.
To maintain Katachi, you must keep you hips very low, to keep a low center of gravity, back arched, look forward, ad push elbows in.
This translates into the Ice-cube Tachiai which is the ideal to be aimed for. If you have the proper Katachi, you will hit the opponent with the forehead and both hands at the same time. The hands will be almost touching and the elbows will be pushed inside.
This way you should always attain Nihonzashi, which is the ideal position to fight from. You must always have Nihonzashi. If you fail to achieve it, you have made a gigantic mistake right at the start. If you don't even Fight for it, you are not doing Good Sumou and Semeruzumou.