An uninitialized variable is one that has never been set to a known value. Attempting to use such a variable is a logical error that will cause a program to act very oddly.
An unqualified name is a reference to a member variable that doesn't specify which object the member variable belongs to. When we use an unqualified name in a member function, the compiler assumes that the object we are referring to is the object for which that member function has been called.
An unsigned char is a type of integer variable; see char for details.
An unsigned int is a type of integer variable; see int for details.
An unsigned long is a type of integer variable; see long for details.
An unsigned short is a type of integer variable; see short for details.
An unsigned variable is an integer variable that represents only positive values (and 0); see the individual variable type (char, short, int, or long) for details.
A user defined data type is one that is, well, defined by the user. In this case, however, user means "programmer". The primary mechanism used to define a user defined type is the class.