Our Timeline (OTL; sometimes called Original Timeline), is how we in the Alternative History community talk about real history. OTL comprises the events, situations and figures that exist in the history we know and live in. The contrast to OTL is called an ATL (Alternate Timeline) or TTL ("This Timeline", that is, the one being discussed); in an ATL something has changed to produce a different setting.
In timelines and discussion, we often refer to OTL to use as a baseline for comparing the historical changes caused by our characters and events. Characters in-universe might talk about OTL if they are aware of changes occurring, such as in "Time Patrol" stories where the timeline is changed deliberately or as an act of criminality.