Final answer:
A weld line is the mark on a molded piece resulting from the meeting of two or more flow fronts during the molding operation.
Step-by-step explanation:
The mark on a molded piece resulting from the meeting of two or more flow fronts during the molding operation is called a weld line. This occurs when molten material flowing from different directions meets and solidifies without fully merging, leading to a visible line on the surface of the molded part. Contrastingly, a flash refers to the excess material that extends out of the mold's intended design, typically thin and easily removed, a gate mark is the point where the molten material enters the cavity of the mold, and a knit line is similar to a weld line but is often used to describe the point where two flow fronts meet when a thermoplastic material is being injection molded. It's important to control these flow fronts properly during molding to minimize weld lines as they may represent weak points in the molded part.