The action of reducing the tightness or firmness of something can be referred to as?

Loosening is the action that directly relates to reducing the tightness or firmness of an object or substance. When something is loosened, it means that tension or pressure is decreased, making it less rigid or able to move more freely. This term is commonly used in various contexts, such as in mechanics (e.g., loosening a bolt), textiles (e.g., loosening a knot), or even in a more abstract sense, as in relaxing a grip or hold.

The other terms present different actions: lubricate refers to applying a substance to reduce friction, kink refers to creating a twist or bend that often increases tightness, and inflate means to fill something with air or gas, which would increase tightness rather than reduce it. Hence, "loosen" is the most appropriate term to describe the action of reducing tightness or firmness.