Dilation is expansion, usually of an opening such as the pupil. When that happens, your eyes look black and the light comes streaming in. Grab your sunglasses!
The word dilation is the noun form of dilate, "to make wider.” Dilation often refers to what happens when a woman goes into labor, and her cervix dilates to let the baby through. Dilation is also used for writing or speech that expands on a topic. Here’s a dilation: funny thing is that the -ate ending of dilate is not a suffix but part of the word, so it should really be dilatation, but no one says it that way.