The smallest frame size encountered when a packet travels from its source to its destination. There can be different data link protocols (layer 2) in a network with differing MTUs. See MTU.
"path MTU." YourDictionary, n.d. Web. 17 August 2018. <http://www.yourdictionary.com/path-mtu>.
path MTU. (n.d.). Retrieved August 17th, 2018, from http://www.yourdictionary.com/path-mtu