Latency

Techopedia explains Latency

In data communication, digital networking and packet-switched networks, latency is used in two major contexts. One represents a one-way trip while the other is a round trip. One-way latency is measured by counting the total time it takes a packet to travel from its source to its destination.
Round-trip latency is measured by adding one-way latency from the destination to the time it takes the packet to return from the destination and arrive back at the source. Unlike one-way latency, round-trip latency always excludes processing time at the destination point. A service called ping is used to measure round-trip latency.

In formal network transmission, the following four elements are involved in latency:

1. Delay in Storage: As data is written on hard disks and other storage devices, a delay occurs in reading and writing to and from different blocks of memory. Processors often consume a lot of time finding the exact location for reading and writing data. Sometimes intermediate devices like switches or hubs also cause delays.
2. Device Processing: Latency is not limited to storage devices but can also be caused by different network devices. For example, when a router receives a data packet, it keeps that packet for a few seconds to read its information and also to write some extra information.
3. Transmission: There are many kinds of transmission media and all have limitations. Each medium, from fiber optics to coaxial cables, takes some time to transmit one packet from a source to a destination. Transmission delays depend on packet size; smaller packets will take less time to reach their destination than larger packets.
4. Propagation: Delays occur even when packets travel from one node to another at the speed of light.