出血是印刷专有名词，为印前作业之一。由于出版品的第一页到最后一页其版面会有些许误差，为避免印刷成品裁切失准或留下白边，因此在设计时，通常会将制作内容（底图、底色）超出实际版面大小。

出血通常比印刷出来的实际物品上下左右各超出3mm ，因为通常裁切的时候不一定可以精准的跟实际尺寸相符合，所以必须预留出血以供裁切。

Bleed is a printing term that refers to printing that goes beyond the edge of the sheet before trimming. In other words, the bleed is the area to be trimmed off. The bleed is the part on the side of a document that gives the printer a small amount of space to account for movement of the paper, and design inconsistencies. Artwork and background colors can extend into the bleed area. After trimming, the bleed ensures that no unprinted edges occur in the final trimmed document.