__private − Address Space Qualifier.
OpenCL implements the following disjoint address spaces: __global, __local, __constant, and __private The address space qualifier may be used in variable declarations to specify the region of memory that is used to allocate the object. The C syntax for type qualifiers is extended in OpenCL to include an address space name as a valid type qualifier. If the type of an object is qualified by an address space name, the object is allocated in the specified address name; otherwise, the object is allocated in the generic address space.
The address space names without the __prefix i.e. global, local, constant and private may be substituted for the corresponding address space names with the __prefix.
The generic address space name for arguments to a function in a program, or local variables of a function is __private. All function arguments shall be in the __private address space.
functionQualifiers(3clc) function arguments declared to be a pointer of a type can point to one of the following address spaces only: __global, __local or __constant. A pointer to address space A can only be assigned to a pointer to the same address space A. Casting a pointer to address space A to a pointer to address space B is illegal.
Function arguments of type otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), otherDataTypes(3clc), and otherDataTypes(3clc) refer to image memory objects allocated in the __global address space.
Variables inside a __kernel function not declared with an address space qualifier, all variables inside non−kernel functions, and all function arguments are in the __private or private address space. Variables declared as pointers are considered to point to the __private address space if an address space qualifier is not specified.
The __global, __constant, __local, __private, global, constant, local, and private names are reserved for use as address space qualifiers and shall not be used otherwise.
There is no generic address space name for program scope variables. All program scope variables must be declared in the __constant address space. For example:
There is no address space for function return values. Using an address space qualifier in a function return type declaration will generate a compilation error, unless the return type is declared as a pointer type and the qualifier is used on the points−to address space.
OpenCL Specification 
The Khronos Group
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page 222, section 6.5 - Address Space Qualifiers