"Sequential art" can be broadly defined as any form of art which consists of discrete elements existing in time; that is, which have been set into a sequence by the artist and are intended to be experienced in that sequence by the audience. Cinema (read broadly as "the moving image"), music, and comics (the medium which the term "sequential art" was coined by Will Eisner to describe) are sequential art forms.
Sequential art joins together quanta of expression.
Without the element of time, musical works would either be many notes all played simultaneously, or one single note. The old saw about music being about "the notes you don't hear" is illustrative here; without time, the duration notes are sounded or the space between them, you can't have melody. You can't have rhythm. You simply have sound.
Cinema is time-dependent in several ways. Firstly, it is, itself, a time-based illusion; a motion picture is nothing more than a series of stills shown so quickly as to exploit persistence of vision and blend them into fluid images. Secondarily, stories told cinematically also exist in time. A film which objectively runs for two hours can tell a story that takes place in a single second or over vast bilennia.
The medium known as comics allows the reader incredible latitude to decide how much time has passed between panels, and how quickly he or she wishes to read them. The time element is provided solely by the audience and is, in fact, the missing ingredient required to make comics understandable as communication at all. Why does this picture of a guy holding a mallet over his head have any relation to the subsequent picture of another man with a lumpy cranium? A certain amount of time must have passed in the space between panels in which the mallet came down and cracked our poor inky example on the noggin. Comics is like blinking really slowly.
All sequential art forms leave room for innovators (or frustrators, depending on your point of view) to play with the audience's perception of time. Sequence can be shattered, with diegetic events unfolding in a way which bears no resemblance to when they must have "really" happened. Notes can be held for long minutes, punched staccaticly or not played at all; John Cage's 4'33" is nothing but time*.
Is it a coincidence that the visual manifestations of sequential art have gained prominence in the same century that brought us quantum physics? Is visual art reflecting our new world view? Or is sequence simply another color on a pallette, biding it's time until it is blended with whatever the next shade may be?
* Okay, Pseudo_Intellectual has shamed me into clarifying that Cage's 4'33" is not really "nothing but time". It is, in fact, an exploration of whatever sounds may occur in that time period. A much more in-depth discussion about it can be found in the appropriate node, and I apologize to any other Cage fans I've offended by my narrow discussion of his work.
Much thanks to Scott McCloud, whose book Understanding Comics helped me toward a greater understanding of visual art.