a reference copy, we only copy a pointer to a shared data block

(of an object) A new object that has the same contents as the object. The contents are not copied, but are the same objects contained in the original object.

when an object contains pointers, a shallow copy only makes a copy of the pointers and not the objects pointed at. For example. A shallow copy of a linked list, will copy only the head pointer, both the original and copied head pointers will point to the same list.

In making a copy of a complex data structure, it is often possible to copy upper levels of data without copying lower levels. If lower levels are indeed shared, it is a shallow copy. See also deep copy (see deep copy).