In the context of digital networks, particularly those transmitting real-time data such as voice over IP (VoIP) or video conferencing, jitter can affect the quality of the service. 

• Packet Delay Variation: Jitter is essentially the irregularity or inconsistency in the time delay between data packets sent over a network. While some delay is expected in packet-switched networks, jitter refers to the variability of these delay times.

• Impact on Real-Time Communications: In real-time applications like VoIP or online gaming, high levels of jitter can result in poor audio and video quality, with effects like choppy audio, momentary silences, or glitchy video playback.

• Measurement: Jitter is typically measured in milliseconds (ms). It's calculated by determining the difference in time it takes for packets to arrive, comparing the expected arrival time to the actual time of arrival.

• Causes of Jitter: Jitter can be caused by various factors, including network congestion, improper queuing, or configuration errors in networking hardware. Variances in route and traffic load can also contribute to jitter.

Managing jitter is crucial for maintaining the quality of service in real-time communications and multimedia applications over IP networks.