1. A temporary storage space which holds data for future processing. The data may be stored on a hard disk, in RAM or on specialised chips such as UARTs. From Linux Guide @FirstLinux

2. Buffering/reblocking program for tape backups, printing, etc. Buffer implements double buffering and can be used to keep backup tapes streaming or printers printing. It can also be used to convert a data stream to a given output blocksize. Buffer uses shared memory to convert a variable input data rate to a constant output data rate. It is typically used in a pipe between a backup program and the tape device, but there are also other applications like buffering printer data in lpd's input filter. From Debian 3.0r0 APT