The endurance or fatigue limit is the maximum value of stress that a polished standard specimen can withstand without failure for an infinite number of cycles when subjected to a completely reversed load. This means that the specimen is subjected to alternating stress that changes from tension to compression and back again, with the same magnitude.

Explanation:
1. Definition of endurance or fatigue limit:
The endurance or fatigue limit is the maximum stress value that a polished standard specimen can sustain without failure for an infinite number of cycles when subjected to a completely reversed load. It is typically determined through fatigue testing, where the specimen is subjected to cyclic loading until failure occurs.

2. Static load:
Static load refers to a constant and unchanging load applied to a structure or component. In the case of a polished standard specimen, applying a static load will not cause failure at any stress level below the endurance limit. However, when the stress exceeds the endurance limit, failure will eventually occur, even if the load is static.

3. Dynamic load:
Dynamic load refers to a varying or fluctuating load applied to a structure or component. Similar to a static load, a dynamic load below the endurance limit will not cause failure in a polished standard specimen. However, if the stress exceeds the endurance limit, failure will eventually occur, even under dynamic loading conditions.

4. Static as well as dynamic load:
When a specimen is subjected to both static and dynamic loads, the endurance limit becomes crucial. If the stress level remains below the endurance limit, the specimen will not fail, irrespective of whether the load is static or dynamic. However, if the stress exceeds the endurance limit, failure will occur, regardless of the type of load applied.

5. Completely reversed load:
A completely reversed load refers to a load that changes from tension to compression and back again, with the same magnitude. In this case, the endurance limit is particularly important. If the stress level remains below the endurance limit, the polished standard specimen will not fail, even with a completely reversed load. However, if the stress exceeds the endurance limit, failure will eventually occur.

Conclusion:
The correct answer is option 'D' - completely reversed load. The endurance or fatigue limit of a polished standard specimen is defined as the maximum value of stress it can withstand without failure, for an infinite number of cycles, when subjected to a completely reversed load. This concept is crucial in determining the fatigue life and durability of materials and components in mechanical engineering applications.

B