This article has been Let's start with a Latin verb, ferre, to carry. Just as in English we give the major inflected forms of the verb (the so called paradigm) in the form present infinitive, past indicative, past participle—for example, eat, ate, eaten—so in Latin the paradigm is given in the form present indicative, present infinitive, perfect indicative, past participle or supine—for example, amo, amare, amavi, amatum.
Now words that end in -ation or -ative or -atory in English are derived from the Latin supine; amatum, for instance, gives us amatory. But derivatives of ferre come from three different words and the paradigm is highly irregular: fero, ferre, tuli, latum. So refer (from referre, literally to bring back) is related to relation (from relatum, brought back); other examples are given in the table. It is notable that words that derive from -ferre are all in common use and need no definition, while several of those that derive from -latum are either obscure or obsolete (and are defined in the table).
Other words that have -atum in the supine but no -at- elsewhere in the paradigm also give words that end in -ation etc. For example, ambulatum (from ambulare, to walk) gives us ambulation and ambulatory.
In two cases English words derive from Latin words in which the infinitive ends in -atare and in which the -at- is therefore reduplicated in the supine; they are dilatare, to spread out, and natare, to swim. The paradigm for the former is dilato, dilatare, dilatavi, dilatatum. So we get dilate from the stem of the infinitive but dilatation from the supine. And natation, swimming, comes from natare ... natatum; “nation” as an alternative, along the lines of “dilation,” would obviously be wrong.
It follows that we should also form “dilatator” from the supine, but although this was in use at one time, for some reason we now use the incorrect form dilator. This is sanctioned by long tradition—Samuel Johnson's dictionary of 1755 has “dilatation” but “dilator.”
Johnson also included “dilation” in later editions of the dictionary, but this obsolete word is different, in more ways than one: it comes from differre (see table) and means delay (compare, dilatory). Perhaps that explains long surgical waiting lists—dilatory dilators.
