"Tidal" and "Tidal Wave"

This is something that's fascinating to me. On CNN today, one of the weather anchors mentioned that it is incorrect to call a tsunami a "tidal wave" because it has nothing to do with the periodic motion of the tides. I thought this was an interesting point, so I decided to look up some definitions to verify that statement. As you might imagine, what I found was shocking. This is perhaps a "bug" in the English language.

Every dictionary definition I can find of "tide" and "tidal" involves periodic ebbing and flowing. In most cases the specific example of periodic motion caused by the gravitational interaction of two massive bodies is mentioned. For example, from Merriam-Webster's definition of the adjective "tidal":

Main Entry: tid·al
Pronunciation: 'tI-d^&l
Function: adjective
1 a : of, relating to, caused by, or having tides <tidal cycles> <tidal erosion> b : periodically rising and falling or flowing and ebbing <tidal waters>
2 : dependent (as to the time of arrival or departure) upon the state of the tide <a tidal steamer>
- tid·al·ly

Because "tidal" modifies "wave" in "tidal wave," then that implies that a tsunami is NOT a "tidal wave."

However, "tidal wave" is a special exception that has its OWN definition:

Main Entry: tidal wave
Function: noun
1 a : an unusually high sea wave that sometimes follows an earthquake b : an unusual rise of water alongshore due to strong winds
2 : something overwhelming especially in quantity or volume <a tidal wave of tourists>

And so while it seems like we should interpret "tidal" as modifying "wave," it is more appropriate to think of "wave" as "modifying" "tidal." That is, a "tidal wave" is an unusual wave that is above and beyond the normal periodic ebb and flow of the tides.

And thus, we get the dictionary definition of tsunami:

Main Entry: tsu·na·mi
Pronunciation: (t)su-'nä-mE
Function: noun
Inflected Form(s): plural tsunamis also tsunami
Etymology: Japanese, from tsu harbor + nami wave
: a great sea wave produced by submarine earth movement or volcanic eruption : TIDAL WAVE
- tsu·na·mic /-mik/ adjective

So even though "tsunami" means "harbor wave," which almost implies a non-periodic wave, somewhere someone decided to translate it as "tidal wave," and thus we have to come up with silly justifications for using the word "tidal" in this sense.

So the weather anchor was probably technically wrong in what she was trying to convey, but she did hit on an interesting language mess...

So if you want to imply something's gentle and periodic, use "tidal." If you want to imply something is impulsive and infrequent, use "tidal wave."