Very often used when speaking of video or audio quality -- defines how much physical space one second of audio or video takes in bits (note: not in bytes). For example: 3 minutes of MP3 audio in 128kbit/sec CBR bitrate takes 2.81 megabytes of physical space (1,024 x 128 / 1,024 / 1,024 x 180 / 8).

Another term that is very often used -- specially with videos -- when speaking of filesize, is the framerate.