"Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs. The most common use regards online gaming when the game doesn't respond in sync with the player's controls, usually due to a slow internet connection, server latency or overworked hardware."
Do you understand what that means? It means that when you say lag then its automatically implied that it is network lag since its an online game. In case of graphic lag the game engine would require you to render a set amount of frames/ticks per second, lets say for example, 30, the lag would be having some frames from the previous second moved to the next one, that is lag, its failing to keep up not ditching totally like its in Spiral Knights. There can't be fps lag in this game. You can get network lag and input lag and sound lag though. So next time instead of saying that your game is lagging when you are talking about your game framerate dropping just say that the game slows down and isn't as smooth, you don't need to talk technobabble.
Oh, and, freezes at random/consistent intervals are called stuttering.
herp derp
yet there will still be people who will use the term incorrectly. Like Kleenex, Q-tips, Band-aids, Vaseline, Poprocks, Coke, MMO, burgers, iPod, Jell-O, Icee, Power Bar, a whole nother, irregardless, youse, pulling a Homer, and Robot Chicken.