維基對這兩個字的說法:
Relating to movement of physical matter
• Slip (aerodynamic), an aerodynamic state in which an aircraft is moving sideways as well as forward relative to the oncoming airflow
Skid may refer to:
• Skid, a type of pallet, a metal, wood or plastic platform for holding machinery or equipment. Some pallets have planks across the bottom level, flush with the floor, but a skid has no planks along the full length or width to form a bottom level on the floor.
For all CSIA followers, a definition has been given by CSIA:
Skidding: the sideways travel of one or both skis across the snow. Necessary for speed control.
The same movement/technique may be called by another term in other systems.
I think the PSIA definition is pretty close to that of CSIA, skidding is against carving, whether or not it's a required skill or an expected movement PSIA does not say it.
CSIA clearly says "necessary for speed control"
Compared to PSIA definition, CSIA does not mention the moving forward part but I think it's implied by default.