di·late / ˈdīˌlāt; dīˈlāt/ • v. 1. make or become wider, larger, or more open: [intr.] her eyes dilated with horror | [tr.] the woman dilated her nostrils. 2. [intr.] (dilate on) speak or write at length on (a subject).DERIVATIVES: di·lat·a·ble adj.di·la·tion / dīˈlāshən/ n.

dilate
A. †relate at length XIV; discourse at large upon XVI;

B. make wider XV; become wider, expand XVII. — (O)F. dilater — L. dīlātāre spread out, f. DI- 1 + lātus wide.
So dilatation XIV; largely superseded by the shorter dilation (XV), formed as if dilate contained the suffix -ATE 3.