The term ”stiff heart” is likely to refer to a heart that does has some impairment in its relaxation function. During the relaxation phase of the cardiac cycle (diastole), the heart chambers fill with blood that is squeezed out during the pumping phase of the cardiac cycle (systole). There are many diagnoses that may be associated with a “stiff” heart- these include hypertension (high blood pressure) and diabetes mellitus for example. “Stiff” hearts can also be seen in elderly patients. Talk with your cardiologist and see what he/she thinks your cause is. The usual treatment is directed at the cause of the diastolic impairment
Cardiologists use this term for a few things, but the most common usage for the term is Cardiac amyloidosis. This is where the muscle is replaced by other proteins making the heart less effective. Has your Doctor indicated any treatments, and what test did you have to diagnose this?
This is really ambiguous because as I said, the term 'stiff' is used by cardiologists many times for different things. It's just a simpler term to use so patients understand what is happening rather than the long technical name. If a heart takes a little longer to relax after contraction, they often tell the patient that their heart is stiff. So I would clarify this with your Doctor, obtain the exact meaning.