Latency

 la·ten·cy

noun

from a general point of view, is a time delay between the cause and the effect of some physical change in the system being observed.

1. 1.

   the state of existing but not yet being developed or manifest; concealment.

   "tension, and the latency of violence, make the greatest impressions"
2. 2.

   COMPUTING

   the delay before a transfer of data begins following an instruction for its transfer.

   "poor performance due to network latency".
3. 
   
4. Latency is the literal time it takes for a packet of data to go from its origination and reach its destination. The measurement of latency is measured in milliseconds. Just 50 milliseconds of latency — a time of less than one-tenth of a second — can result in poor network and application performance.
5. What is delay and latency?

   Delay is the time it takes for data to travel from one endpoint to another. Latency, though, may be one of two things. Latency is sometimes considered the time a packet takes to travel from one endpoint to another, the same as the one-way delay. More often, latency signifies the round-trip time.

Generally, an acceptable latency (or ping) is anywhere around 40 – 60 milliseconds (ms) or lower, while a speed of over 100ms will usually mean a noticeable lag in gaming.