紫斑（purpura、紫瘢）是指在皮肤上出现的紫色血块。

3毫米～ 10毫米的紫色血块。其他大小的红色或紫色的血块请参见：瘀点（petechiae，小于3毫米），瘀斑（ecchymosis，大于10毫米）。

Purpura is a condition of red or purple discolored spots on the skin that do not blanch on applying pressure. The spots are caused by bleeding underneath the skin usually secondary to vasculitis or dietary deficiency of vitamin C (scurvy). They measure 0.3–1 cm (3–10 mm), whereas petechiae measure less than 3 mm, and ecchymoses greater than 1 cm.