Well, when the pupil widens, it's called pupil dilation. I don't think there's a specific term for pupil dilation that happens due to attraction. For reasons deep in our evolutionary past, romantic or sexual attraction seems to activate the sympathetic nervous system (also known as the "fight or flight" response). Other features of this response are "butterflies" in the stomach, sweating, blood rushing to the face, shaking hands, and dry mouth. So unexpectedly running into the object of your affection (if you're still in the "major crush" phase) can make your body respond in the same way it does if you unexpectedly run into a mountain lion. In the physical sense that is, if you're actually afraid of the person you're attracted to, stay away!

According to a study I saw on TV, most people are not consciously aware of another person's pupil size unless they're trained to look for it. But people seem to notice it unconsciously and find it attractive. Any idea how this test could be done? Thanks for asking.

Great question! No one knows for sure the exact "chemistry" behind this, but feelings of "pleasure" or "desire" are coincident with measurable changes in small peptide hormones and neurochemicals that are released quickly into the bloodstream. The phenomenon of pupil dilation is intriguing because some of these small peptide hormones can trigger the same dilation response as light (actually, the light gets converted to chemical messengers in the cell). So in effect, the cells interpret the signal the same way, regardless of the stimulus (light or the hormone). See if you can dive into some reading and make a list of the many peptide (and other) hormones -- I bet you already have heard about endorphins ("runners high") and serotonin!