A timeline is a list of important events arranged in the order in which they happened. You might write out a timeline of Civil War battles when you're studying for a big history test.
Timelines are often used in history textbooks and biographies — they explain what happened during a certain period of time or to a particular person, starting with the earliest event and moving forward through time. Your own personal timeline, for example, might begin with your birth. Many timelines are represented graphically by an actual line that's dotted with points representing important events.