Jitter
Variation in packet delay over time, so packets of one stream arrive at uneven intervals; it harms real-time traffic such as voice and video more than bulk data transfer.
Jitter is the variation in delay between packets that belong to the same stream. If one packet arrives promptly and the next is held in a queue, the stream is uneven even when the average delay looks acceptable. The usual causes are congestion and queuing at routers and switches, packets taking different paths, and competition with other traffic on shared links. RFC 3550, the Real-time Transport Protocol specification, defines a way to estimate it, and RFC 3393 describes the related metric of IP packet delay variation.
Jitter is one of the performance metrics the ISC2 exam outline lists for Domain 4, beside bandwidth, latency, throughput and signal-to-noise ratio. It differs from latency in what it measures: latency is how long delivery takes, and jitter is how much that time varies. File transfers tolerate it, because data is reassembled before use, but voice over IP and video play packets as they arrive, so uneven arrival produces gaps and distortion. Receivers smooth it with a jitter buffer, which trades a small fixed delay for steadier playback, and networks limit it with quality of service prioritisation.
Exam relevance: a scenario describing choppy calls on a link with enough bandwidth is likely to point to jitter. Candidates are expected to keep jitter, latency and bandwidth distinct.