What is the definition of strain/loading rate dependence/sensitivity of compressive strength of brittle materials? Is it general strain/loading or it is specific for the brittle materials?
The quote (from Wikipedia) below answers your question (in general). Otherwise, you shall provide specific example to obtain meaningful response.
"The strain rate at some point within the material measures the rate at which the distances of adjacent parcels of the material change with time in the neighborhood of that point. It comprises both the rate at which the material is expanding or shrinking (expansion rate), and also the rate at which it is being deformed by progressive shearing without changing its volume (shear rate)."
Keep shearing in mind, while it is not a "brittle" material only phenomenon, the brittle mater material is weaker in resisting shear stress/force, thus the strain/load rate is more of an interest of brittle material.