The pattern buffer was the part of a transporter system that held the information or material compromising the subject being transported.
When the USS Jenolin crashed on the surface of a Dyson sphere, Montgomery Scott rigged the pattern buffers of the ship to hold the patterns of himself and Matt Franklin until they could be rescued; such a rescue took seventy five years until the USS Enterprise-D discovered them. By that point, one of the pattern inducers had failed, and Franklin's pattern was too degraded to rematerialize. (TNG episode: "Relics")
In an alternate timeline where Montgomery Scott rescued James T. Kirk before he could be absorbed into the Nexus, it was discovered that the proper flow of space-time could only be restored by returning Kirk to the energy ribbon. His pattern was dematerialized and stored in the pattern buffer before the ship approached the Nexus in order to reduce the time necessary. (Star Trek novel: Engines of Destiny)