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.
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?