Argthtjtr

In the medical profession we use the terms dilatation and dilation with great frequency. Dilatation is defined as a region of dilation, an area of abnormal enlargement, or the surgical enlargement of a region (noun describing the verb). Dilation is defined as the act of dilating (stretching out), the state of being dilated, and confusingly: dilatation. Both definitions cross-refer to one another.

And, yet I've never gotten a sufficient answer as to why we use one vs. the other. Some, try to claim that dilatation is an active process (like dilatation and curettage), and dilation is a passive process. But, we dilate people's pupils by giving medications. Or using balloon angioplasty we dilate a stenosed blood vessel.

Other sources claim that dilation is a uniform enlargement compared to dilatation. And, other sources claim that there is, in fact, no difference between the two.

In research of this question I sought out clues from the etymology And, I found that dilatation is the older form. That dilation was a mistaken assumption in the 1590s that -ate was the Latin Verbal suffix (and not part of the root).

So, my question: Is there really a difference between the two terms? Or are they truly interchangeable as the etymology would suggest? Has there been a shift in meaning within, say, the Engineering world?

There is a paper that addresses this exact question, titled appropriately enough "Dilation vs. Dilatation: A Brief History" (Journal of Urology 1992: Vol. 147, 1682; Bloom, Mory and Hinman) which appears to conclude that there is no difference.

The verb dilate is first recorded in English by Gower in 1393

...

The verb dilatate is recorded only once (1613) and is described as an obsolete synonym for the verb dilate. Thus one of two competing words fell into obsolescence. The nouns dilatation and dilation are more difficult to distinguish. They seem to be synonymous [Emphasis mine]. Chronological priority belongs to the former (1386).

...

Dilation seems to be the result of an orthographic shortcut, wherein a repetitious syllable (ta) was dropped.

Then the authors continue to talk about the etymology of "dilate" and "dilatate" before saying:

It is for this reason that the OED states that the etymologically correct form is the earlier dilatation and that dilation is formed "improperly." Today we would prefer to describe this as erroneous or irregular formation. In spite of the linguistic accuracy of dilatation, current usage favors dilation. It pairs more evenly with the verb dilate. In addition, there is precedent in urology for dropping staccato-sounding extra syllables in words or combinations, such as in urine analysis versus urinalysis.

Historically, dilatation and dilation existed as competing forms and have been used interchangeably. Today we are witnessing the eclipse of dilatation in favor of dilation, a perfectly normal linguistic occurrence. Attempted distinction between the two, namely that of an action versus a condition, seems arbitrary and without historical evidence or support. [Both emphasis mine]

So the conclusion of Bloom, Mory and Hinman is fairly unambiguous: there is no difference. "Dilatate" is a historical variant of "dilate", and the two words are interchangeable.

In my experience, pupils are dilated (either pharmacologically or due to neurological impairment) while urethras and uteruses are manually or instrumentally stretched in a process called dilatation. This Ngram seems to reflects this usage and demonstrates what the OP reports that dilatation is an older term. The next Ngram looks at ventricular dilatation vs. ventricular dilation. Dilation appears in the ascendancy while that extra "T&A" is losing ground. Finally, "dilatation & curettage, of course, is an ancient surgical procedure. Perhaps it provides some linguistic inertia that keeps the older term in play.