Jitter is the level of variability in the latency of messages across a network or other communication channel. For example, suppose you are on holiday in a remote part of the world and send a postcard each day to your mother; on average the letters take seven days to reach her, but some take only five and some as much as two weeks. This variability, between 5 and 14 days is the jitter. The times involved are of course far shorter for most networks, but the principle remains the same. See in the terminal session below a series of 'pings' to a server from a laptop; the latency is around 20ms, but varies from 18.682 ms to 53.564 ms, more than twice the average.
When thinking about the quality of a network, alongside jitter one normally considers latency (the end-to-end delay) and bandwidth (how much information can be passed per second). In addition, in rural areas or other places with poor connectivity, glitches are also crucial, short periods when the connectivity breaks entirely.
> ping hcibook.com
PING hcibook.com (185.199.220.49): 56 data bytes
64 bytes from 185.199.220.49: icmp_seq=0 ttl=56 time=18.873 ms
64 bytes from 185.199.220.49: icmp_seq=1 ttl=56 time=18.682 ms
64 bytes from 185.199.220.49: icmp_seq=2 ttl=56 time=26.621 ms
64 bytes from 185.199.220.49: icmp_seq=3 ttl=56 time=53.564 ms
64 bytes from 185.199.220.49: icmp_seq=4 ttl=56 time=19.080 ms
64 bytes from 185.199.220.49: icmp_seq=5 ttl=56 time=22.872 ms
Used in glossary entries: bandwidth, glitches, latency
Bandwidth, latency, and jitter. Figure 12.7 from Network-Based Interaction