In the terms of a game, for example, many people would call 12 frames per second “laggy”. However, the computer is accepting, and processing the inputs just as quickly as it usually would- the fact that the “effect” from these inputs is only seen at 12 frames per second can hardly be called a result of lag.

In terms of say, a edit box that takes a second to register input, it depends on the context. Following the definition, the only thing that is falling behind anything else is the actual display as opposed to what the user “thinks” should be there. Therefore, “lag” as experienced on a local machine is not really measurable except by perception. What one person considers “lag” another might consider perfectly acceptable, and in both cases the result is the same.

With a network, however, there is a measurable delay. the very concept itself was more or less used originally with regard to “ping” times, or, more precisely, delays based on distance from a server. If PlayerA is a mile from the game server and player B was on the other side of the continent, then PlayerB was always “lagged” behind playerA, and there was nothing that anybody could do about it. Additionally, spikes and dips in connectivity and speed as well as the condition of the route being taken by the packets could mean that the various packets that are being sent by the server will take a different amount of time to reach each client- oftentimes a complete loss of packets, or duplicate packets (As a result of the server retransmitting the packets again as per the TCP protocol). This is a physically measurable quantity of time that doesn’t rely on a persons perception and there is no doubt wether there is any slowdown.

Case and point- quite some time ago, I played through several games at a speed of only 18-25 fps, and I found it quite playable. However, many people would call it “laggy”. The fact is, when the concept being used has a strict definition and it’s application relies purely on a perceptual measuring of that definition the term itself becomes nearly meaningless.

‘it’s not “lag” because there is no actual lag between your actions and what you see on screen’

Erm.. the term “lag” means a time delay – pressing a key and seeing the actions a brief time later *is* a lag; the effect is lagging behind the action.

‘the term “lag” was originally used to indicate the time difference between when you pressed a key and when the server acknowledged you pressing that key’

I think you’re talking specifically about “network lag” there, not “lag” in general. Best not confuse “lag” with automatically meaning network context.