The nucleus of this article is the notion that using the term tidal wave to describe a tsunami is incorrect because tsunamis are not caused by the gravitational influence of the moon and sun. While I will take the opportunity here to pull together the arguments I have given in various places to the effect that tidal wave is in fact a perfectly good term for a tsunami, I would also like to step back and explore the usage of the term tide and various terms derived from it.
Before I delve into this, I would like to offer a disclaimer: I don't claim to have researched the complete history of any of these terms in detail. The various claims I will make in what follows are based on a reasonable amount of digging through sources old and new, and I believe they are substantially correct. I will informally cite evidence where I can. Further detail and earlier drafts of some arguments may be found on Quotations:Tidal wave, Category talk:English words affected by prescriptivism, Talk:tsunami and Talk:tidal wave. Nonetheless, I can't say any of this is definitive. Please imagine any seemingly definitive claim prefaced by "As far as I can tell ..." or "From my incomplete research" or similar qualification.
The word tide itself is cognate with German Zeit, Dutch tijd, Swedish tid and so forth, all meaning time. This meaning survives in yule tide  and perhaps to a lesser extent in tidings. However, the use of tide in reference to the sea goes back at least to Chaucer . This is significant because, while (I believe) it was well known at that time that the ocean tides bore some relation to the moon, the modern theory of gravitational influence certainly wasn't around yet. To state the obvious, tide in the sense of "ocean tide" could not have originally been conceived in terms of gravitational influence.
It would be most interesting and highly relevant here to understand just how tide in the sense of time came to apply to the ebb and flow of the ocean. If I had to guess, the link would be that the level and flow of the water directly affected shipping schedules. A large ship might not even be able to leave harbor if the water was too low, and all vessels are at an advantage when traveling with the prevailing current. Thus a ship would generally sail when the water had just reached its peak and was beginning to flow back out to sea. The usual term even today is to sail with the tide. Even if this is not quite what happened, there is clearly a strong lingustic link of some sort between opportune times to sail and the sea conditions at those times.
As we know now, the tide in this scenario is determined primarily by the moon and by the topography of the ocean floor, but it is important to keep in mind that not all harbors have easily predictable tides , and that from a practical point of view, tides may also be strongly affected by the weather and other factors . In other words, the tides experienced as a practical day-to-day matter may or may not have borne a recognizable relationship with the moon. Up until the theory of gravitational influence was worked out (Newton published his explanation in 1687 ), ocean tides would have been understood as the ebb and flow of the ocean at a particular location without reference to cause, and as local, not global phenomena.
This view goes a long way toward explaining the existence of terms like rip tide, storm tide, hurricane tide, and tidal wave (in the sense of a large and destructive surge), none of which is caused by gravitational influence, but all of which have to do with the flow of the ocean.
It is also significant that figurative uses of the word "tide", both in live metaphors and in idioms such as "rising tide", "the tide is turning" or "swimming against the tide" accord with the local view of the sea ebbing and flowing against the shore. As figurative use depends on literal use to carry the metaphor, this strongly implies that ebb and flow have been the primary basis of the literal senses of tide, even to this day.
Modern dictionaries, however, tend to emphasize the gravitational aspect of tides. I would suspect that this is because the gravitational theory has such a prominent role in predicting local tides — a matter of great practical importance — that the tides which it cannot predict have come to be regarded as not being proper tides at all.
This shift in meaning is just a particularly notable case of the very common process of specialized fields appropriating a common term and giving it a precise meaning. In many cases, intimate knowledge of a field reveals distinctions that, for various reasons, are not made in common usage. For example:
- In common usage, acceleration means "increase in speed", while in physics it means "change in the velocity vector", which may include decrease in speed, change of direction with no change in speed, and so forth.
- In common usage, square root means "positive square root", but mathematically a square root may be any complex number, including negative and imaginary numbers.
- Common usage does not distinguish weight from mass, while physics does.
- Composers and musicians distinguish vibrato from tremolo, but common usage doesn't.
Because a scientific definition in particular is carefully formulated and often based on a more detailed understanding than that behind common usage, scientific definitions are often blessed as "correct". From a linguistic point of view, this is unwarranted. If large numbers of speakers consistently make the same "error", it's not an error.
Linguistics notwithstanding, the tendency to give scientific definitions special status is strong enough that the scientific definition may be taken as the correct definition and even used as grounds for declaring long-standing related usages "incorrect." Thus it is claimed that a storm tide isn't really a tide because tides are (by definition!) caused by gravitational influence.
This leads to a curious state of affairs in which rip tide, storm tide, hurricane tide and the usual sense of tidal wave are all considered incorrect by appeal to the modern definition of tide. This is a classic case of the tail wagging to dog. Rather than puzzling over why people insist on using all these terms incorrectly when science now knows what causes regular tides, it seems more sensible to work from the premise that the modern definition of "tide" is the misnomer (albeit a useful one) and the rest of the usages are (also) correct.
This doesn't mean that the modern definition is wrong. It's quite useful. It's just not the whole story. As it stands, the more general sense of tide is nearly if not completely lost in the case of tide itself, but alive and well in related terms such as rip tide, or even in such phrases as the tide is going out, high tide, tide pool and so forth.
The case of tidal wave is particularly interesting, though. The term seems to date to the 1800's, and my understanding is that the narrow sense of crest of water driven by gravitational influence came first, doubtless accompanied by tidal accelleration and tidal force, which are so completely divorced from ocean currents that they are routinely used in reference to black holes and other astronomical objects.
Nonetheless, tidal wave has evidently been "misused" throughout most of its existence. There seem to be two competing analyses, with origins that I hope will be obvious by now:
- A large-scale phenomenon caused by the gravitational influence of the moon and sun.
- A local phenomenon in which the ocean surges and ebbs, just like a normal tide but quickly and generally to a higher level.
Whether or not the first sense was intended as the original "correct" meaning of tidal wave, the second usage persists because a "tidal wave" in the "large surge" sense is something tangible and further, it fits with the rest of the existing broad senses. Thus a term which may well have been coined as a narrowly-defined scientific term has been co-opted back into the broadly-defined family whence it came.
Interestingly, tsunami is also understood in two distinct ways. To oceanographers, it is a long-wavelength wave in the ocean at large. To the rest of us, it's a mass of water surging up on the shore.
: Thanks to Hippietrail for pointing this out.
: E.g., from the prologue of the Canterbury Tales, describing the shipman:
- But of his craft [skill] to reckon well his tides,
- His streames and his strandes him besides,
- His herberow [harborage], his moon, and lodemanage [piloting],
- There was none such, from Hull unto Carthage
Notably, the other uses of tide in the Cantebury tales appear to refer to time, as in morning-tide, or in the sense of betide.
: For example, near the Panama Canal "Cristobal has an irregular tide varying from two highs and two lows to one high and one low each lunar day - with all possible intermediate variations" (see here for more detail)
: Thus Joshua Slocum's statement (also on Quotations:Tidal wave) that
- Currents on this coast are greatly affected by the prevailing winds, and a tidal wave higher than that ordinarily produced by the moon is sent up the whole shore of Uruguay before a southwest gale, or lowered by a northeaster, as may happen.
: Again thanks to Hippietrail for finding this.