Do an image search for 'timeline' and you will see many examples of a timeline diagram. It's a long line that represents a period of time, and has lots of marks along the line representing different dates within that time period, where notable events happened. When people say timeline, they don't mean the diagram, they just mean the stretch of time that has notable events happening all the way along it. But they often imagine the diagram, because that's a good representation of the meaning.