That's one. However, pupils may dilate due to some other reason. Here's Wikipedia:

Pupillary response is a physiological response that varies the size of the pupil, either resulting in constriction[1] or dilation (expansion), via activation or deactivation of the iris dilator muscle. The response can have a variety of causes, from an involuntary reflex reaction to exposure or inexposure to light — in low light conditions a dilated pupil lets more light into the eye — or it may indicate interest in the subject of attention or indicate sexual stimulation.[2] The pupils contract immediately before someone falls asleep.[3] A pupillary response can be intentionally conditioned as a Pavlovian response to some stimuli.[4]