(1) Resembling fibrin. (2) A deeply or brilliantly acidophilic, homogeneous, proteinaceous material that: 1) is frequently formed in the walls of blood vessels and in connective tissue of patients with such diseases as disseminated lupus erythematosus, polyarteritis nodosa, scleroderma, dermatomyositis, and rheumatic fever; 2) is sometimes observed in healing wounds, chronic peptic ulcers, the placenta, necrotic arterioles of malignant hypertension, and other unrelated conditions. [fibrin + G. eidos, resemblance]