means The scientific study of diseases — their causes, mechanisms, and effects on the body — often involving the examination of tissues, organs, and bodily fluids to diagnose what's gone wrong.

from From the Greek 'pathos' (suffering, feeling) joined to '-logia' (the study of) — so literally 'the study of suffering.' That same 'pathos' runs through a whole family of words about feeling and ailment: sympathy (suffering-with), apathy (without feeling), empathy. Medicine borrowed it through Latin and into English by the 16th–17th century, turning the study of suffering into the study of what causes it.