From The Art and Popular Culture Encyclopedia
The past is the portion of the timeline that has already occurred; it is the opposite of the future. It is also contrasted with the present. It is also regarded as the conglomerate of events that happened in a certain point in time, within the Space-time continuum. The aforementioned conception is closely related to Albert Einstein's relativity theory.
The past in philosophy
According to presentism, the past doesn't exist, but all sciences study virtually the world's past, more or less far. Humans have recorded the past since ancient times, and to some extent, one of the defining characteristics of human beings is that they are able to record the past, recall it, remember it and confront it with the current state of affairs, thus enabling them to plan accordingly for the future, and to theorise about it as well.
In classical physics the past is just a half of the timeline. In special relativity the past is considered as absolute past or the past cone). In Earth's scale the difference between "classical" and "relativist" past is less than 0.05 s, so it can be neglected in most cases.
In other fields