What is the difference in "Inner Diameter" and "Drift Diameter"?

Inner diameter: It is the diameter obtained by subtracting the thickness of the wall from the outer diameter of the object. It is the nominal diameter

While,

Drift diameter: It is the diameter of an object (pipe or bolt) which is the guaranteed diameter, less than the nominal diameter. It is usually 3.2 mm less than the nominal diameter

Explanation:

Inner diameter: It is the diameter obtained by subtracting the thickness of the wall from the outer diameter of the object. It is the nominal diameter

While,

Drift diameter: It is the diameter of an object (pipe or bolt) which is the guaranteed diameter, less than the nominal diameter. It is usually 3.2 mm less than the nominal diameter