In theory, any mechanical device that is used to wash another object could be called a "washing machine". However, the term (at least in the US) is almost exclusively used for machines that wash clothes.
Machines that was dishes and silverware are "dish washers". A Machine that washes automobiles is a "car wash". But a machine that washes clothes is just a "washing machine".
How did that generic term come to be so completely identified with a specific type of washing machine?