undefined

(not comparable)Lacking a definition or value.

неопределённыйneopredeljónnyj

(not comparable)That does not have a logically sound meaning within a specific formal system and is thus not assigned an interpretation.

The result of division by zero is undefined.

Within the set of real numbers, the square root of a negative number is undefined.

неопределённыйneopredeljónnyj

(not comparable)That behaves unreliably due to dependence on an uninitialized value or inconsistently ported code.

In C, relying on a variable's value right after a malloc call is undefined behvaior.