I don't have a scientific explanation, but I believe it would be the time between when you press the button and when something happens. Lag is more or less subjective. I think according to this thread that the amount of lag depends more on which platform you love or hate than anything else! I suppose you could say that technically there is always at least a little bit of lag.
The 3 platforms discussed here handle lag differently. WP & iOS have animations which start pretty much instantly when you tap the button. Android doesn't, at least it didn't in the past. So when you tap a button on WP or iOS, you will see the animation right away. However, there is still some time until the desired action takes place. On Android, when you tap the button, nothing happens, at least by appearance. But after a bit of time the desired action will take place. Often, even if the time from tapping the button to the app opening is the same, you will sense that Android lags because nothing happens right away. But if you have similar grade phones and do the same action side by side, you will see that the total time required is often similar. Put a Nexus 5 beside an iPhone 6 or any WP and do the same thing, and the Nexus 5 will usually be just as quick. In fact, from what I've seen, it will often finish first. But the iPhone or WP will appear faster because something happens right away.
The interesting thing is that this perception is actually more important than it seems. I guess you could say that we're being entertained by the animation! This was explained to me by a Microsoft MVP and WP fan, who now interestingly enough uses Android. However, he doesn't use Android because he likes it better. He switched because of a specific capability he needs that WP doesn't have.
So after that long explanation, I'll sum it up like this: what we call lag is more or less subjective. :winktongue: You're right; it is a catch phrase we throw around.
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