The ANSI C standard supports the concept of the tentative definition. Any external data declaration that has no storage class specifier and no initializer is considered a tentative definition. If the identifier declared appears in a later definition, then the tentative definition is treated as if the extern storage class specifier were present. In other words, the tentative definition becomes a simple referencing declaration. 

If the end of the translation unit is reached and no definition has appeared with an initializer for the identifier, then the tentative definition becomes a full definition, and the object defined has uninitialized (zero-filled) space reserved for it. For example,

int x;
int x;           /*legal, one copy of x is reserved */
int y;
int y = 4;       /* legal, y is initialized to 4 */
int z = 5;
int z = 6;       /* not legal, both are initialized definitions */

Unlike ANSI C, C++ doesn't have the concept of a tentative declaration; an external data declaration without a storage class specifier is always a definition.