Latency
The time data takes to travel from source to destination, measured one way or as a round trip; it is a delay, distinct from bandwidth, which is a measure of capacity.
Latency is the delay between data being sent and being received. It can be measured one way or as round-trip time, the interval between sending a packet and receiving the reply. Several delays add up to it: propagation, the time the signal takes to cross the physical distance; serialisation, the time to place the bits onto the link; processing at each device on the path; and queuing when links are congested. Distance sets a floor that no upgrade removes, which is why satellite communications through geostationary orbit commonly show high latency, and why a content distribution network places content closer to users.
Latency is one of the Domain 4 performance metrics in the ISC2 exam outline. It is easily confused with bandwidth, how much data a link can carry per second, and with throughput, how much actually gets through. A link can have plenty of bandwidth and still high latency. Variation in latency is a separate metric, jitter. Security controls add latency as well: inspection by firewalls and proxies, encryption, and routing remote traffic through a central VPN gateway all add delay.
Exam relevance: questions in this area tend to turn on telling latency apart from bandwidth and jitter. A scenario describing slow interactive response over a high-capacity link is likely to be describing latency.