I know what the "tidal lag" is, but why is it called a "lag?"?

Question

Is it because to an observer, the moon crosses the upper meridian about 40 minutes before high tide?

Or is it because as seen by sidereal observers, the high tide arrives later than the moon?

Answer 1

If the earth was not spinning, there would be a bulge in the ocean directly below the moon.  Because the earth is spinning the tidal bulge moves around the globe because of an unbalanced force trying to move the bulge directly under the moon (where it would always be if water had zero mass and inertia).  Of course, with zero mass there would be no attraction but the point is that inertia retards the water resulting in a lag.

Answer 2

I suspect it is because of the first suggestion rather than the second, since we haven't gotten any input from sidereal observers lately.  ;-))  By the way, the two statements you've suggested are really equivalent, except for the "sidereal" position of the second observer.

The high tide in most places is about 40 minutes after the time that the moon crosses the meridian.  This does actually vary somewhat from place to place, and is one of the parameters used in creating tide tables that are used by fishers and mariners.