Each disease, type of disorder, or clinical trial can have its own definition of a partial remission. For example, a partial remission for cancer may be defined as a 50% or greater reduction in the measurable parameters of tumor growth as may be found on physical examination, radiologic study, or by biomarker levels from a blood or urine test.
Each disease, disorder, or clinical trial has its own specific definition of partial remission.  For example, in cancer, it might mean a 50% decrease in tumor size as determined by physical exam, imaging, or blood/urine tests.