Burstiness is a characteristic of communications involving data that is transmitted intermittently -- in bursts -- rather than as a continuous stream.
A burst is a transmission involving a large amount of data sent in a short time, usually triggered as a result of some threshold being reached. When bursts are a repeated, dependable behavior in some type of transmission, the subject in question is said to exhibit burstiness.
The tendency for communications to be bursty is an important factor in the design considerations of the hardware manufacturers make for these communications. For example, a large supply of fast RAM can replaced by lower cost memory with a cache, saving money while potentially having little to no effect on the user experience.
Burstiness is sometimes caused by the nature of the data being communicated. Ethernet, for example, is inherently bursty. Computer architectures that rely more heavily on cache than sustainable bandwidth are typically bursty as well, showing much higher bandwidth until the cache is exhausted and data must be retrieved from sources outside the hardware where the chip resides. Mp3 players have often combined slower storage with a faster cache this way. This type of architecture can prove ineffective if cache memory often fails to contain the desired information.