Latency is the delay measured between the time a frame first starts to leave the source device and the time the first part of the frame reaches its destination. 
A wide variety of conditions can cause delay as a frame travels from source to destination: 
 Media delays caused by the finite speed that signals can travel
 through the physical media. 
 Circuit delays caused by the electronics that process the signal
 along the path. 
 Software delays caused by the decisions that software must make
 to implement switching and protocols. 
 Delays caused by the content of the frame and influenced by the  frame switching decisions to be made. For example, a device  cannot route a frame to a destination until the destination MAC address has been read