A facility of some programming languages where the TypeSystem can be intentionally subverted, typically to treat values of one type as if they were another.

For example, the following C struct...
 union {
   unsigned int bits;
   float number;
 }
...defines ''bits'' and ''number'' so they share the same memory.  Thus, assigning a value to ''number'' results in a value being in ''bits''.  This can be usefully exploited to determine (for example) whether ''number'' is negative or not by using ''bits'' to examine the bit pattern of ''number''.