Synesthesia is a condition in which experiencing something in one sense modality automatically triggers another sense modality. So, for example, for some people hearing certain sounds or music might trigger an experience of seeing colors as well. Synesthesia is relatively rare, and it is not a medical disease, rather it is a variation on the normal range of human sense experience. In people with synethesia, it is likely that what triggers the phenomenon is neural pathways that allow cross talking between two different sense centers in the brain. Many famous writers, artists, and composers have had synesthesia to one degree or another. Because synesthesia is genetically based and ground in the anatomy of circuitry in the brain, you cannot "get" it if you do not have it.
Seeing things for a few seconds that are not really there is probably not synethesia, unless what you are seeing is something like what I describe - for example, colors in reaction to hearing a sound. Visual hallucinations would be the technical description for seeing something that is not there. Occasional visual hallucinations happen to many people, however if they are occurring frequently you should get in to see your doctor
to make sure this is not part of a larger problem.