"act of dilating," 1590s, formed from dilate on the mistaken assumption that the -ate in that word was the Latin verbal suffix (it is instead part of the stem); the proper form, dilatation, is older (c. 1400).
late 14c., dilaten, "describe at length, speak at length," from Old French dilater and directly from Late Latin dilatare "make wider, enlarge," from dis- "apart" (see dis-) + lātus "broad, wide, widespread, extended" (see latitude). Meaning "expand, distend, spread out, enlarge in all directions" (transitive) is from early 15c.; intransitive sense of "spread out, expand, distend" also is from early 15c. A doublet of delay. Related: Dilated; dilating.
c. 1400, dilatacioun, "act of expanding, expansion," especially "abnormal enlargement of an aperture of the body," from Old French dilatation and directly from Late Latin dilatationem (nominative dilatatio) "a widening," noun of state from past-participle stem of Latin dilatare "make wider, enlarge," from dis- "apart" (see dis-) + lātus "broad, wide, widespread, extended" (see latitude). Also in Middle English "amplification in discourse" (late 14c.). In gynecology dilatation and curettage is by 1896.