What is the definition of unsigned() function in VHDL?
A type conversion isn't a function, it's a basic operation. You don't get to peer into the innards of basic operations. See IEEE Std 1076-2008 5. Types, 5.1 General, paragraph 3. 

There are implicit type conversions, such as between universal integer and integer. That you're asking says you're interested in an explicit type conversion.

9.3.6 Type conversions


  A type conversion provides for explicit conversion between closely related types.
  ...
  Explicit type conversions are allowed between closely related types. In particular, a type is closely related to itself. Other types are closely related only under the following conditions:
  
  - Abstract numeric types—Any abstract numeric type is closely related to any other abstract numeric type.
   - Array types—Two array types are closely related if and only if the types have the same dimensionality and the element types are closely related  
  
  No other types are closely related.


Not to be confused with a conversion function.