Since the first day a human being made a drawing on a cave wall trying to visualize how the hunt went, or should go, there have been sequences. It is just a way of telling a story. It has been implemented in all cultures. From the primitive caveman to the royal Egyptian tombs.
Is there a definition possible? In its most simple form one could say it is a time laps shown in more than one picture. Like for instance four pictures of a tree: one in springtime, one in summer, one in the fall, and the last in wintertime. But does a sequence really has to have a time laps? I don't think so. At the left you can see an example where there is an impression the three pictures are made in the same split second but from a different object or perspective. I use the word impression as the viewer unconsciously expects everything is done with one camera when looking at the photographs. Whether in reality it is done with one or more cameras is not important.
So a sequence can be a movement in time or space by either model or artist (camera) were there is a suggestion of unity between the images. And what about the amount of pictures? Does a sequence consist of at least 2 pictures? Not in my point of view. The twentieth century started with an art movement unlike all others: Cubism. For the first time painters rearranged different points of view in one painting. So from then on it was no longer necessary for artists to finish a work of art in the same perspective as they started. Cubism liberated them from physical and mental boundaries nobody expected could be suppressed.
Les Demoiselles d'Avignon
1907, Pablo Picasso
Perhaps the most well known photographer making use of photographic sequences is Muybridge. To have a better understanding of the movement of humans and animals he photographed them in a very short time laps as for example ten pictures in one second. The result looks like a filmstrip.


Woman Emptying a Bowl of Water and
Paralytic Child on All Fours,
1965, Francis Bacon
Francis Bacon has made a wonderful work
of art inspired by one of Muybridge’s
sequences. Looking at the painting (left)
as well
as the original photo sequences by
Muybridge you could argue they are both
more a kind of ‘stop-motion’ then a
narration in time or space.
Of course many painters also made
sequences with more than one painting.
Bacon did this many times with great
success, as can be seen at the right.

Three Studies for Figures at the Base of a Crucifixion
1944, Francis Bacon
Many Pop Artists felt the need to work with more than one image. Lichtenstein did this using enlarged cartoons for his work.
As I Opened Fire
1964, Roy Lichtenstein
Andy Warhol reproduced a lot of images en used them in all kinds of ways. And although for instance his ‘Marilyn Monroe’ consists of ten different colored images, there is no shift in time or perspective by either Monroe or the artist (camera). So you could call this a work of ‘multiple images’, rather than a sequence.

Marilyn Monroe. 1967 by Andy Warhol