Which term is used to describe reduction in thickness caused by a blunt object?

The term that describes the reduction in thickness caused by a blunt object is "dent." A dent occurs when a force is applied to a surface, causing it to deform inward without breaking through. This type of damage is characterized by a depression or indentation in the material, and it typically results in a change in the surface profile while retaining the overall integrity of the object.

On the other hand, "crack" refers to a break in a material that can compromise its structural integrity, while "score" implies a shallow cut or mark made on a surface, often a result of a sharp object rather than blunt force. "Gouge," meanwhile, describes a deeper removal or displacement of material, often leaving a significant cavity or groove, rather than just an indentation like a dent. Hence, the correct term for the phenomenon described in the question is a dent.