Hi Janet,
Yes, the click to open rate (CTOR) is a measurement of unique clicks by unique opened emails.
For example, say you send 105 emails of which 100 get delivered, 20 have unique opens and 5 receive unique link clicks within the email, your statistics would look something like:
Unique Open rate: 20% (unique opened / delivered)
Unique Clickthrough Rate: 5% (unique clicked / delivered)
Click to Open Rate: 25% (unique clicked / unique opened)
I hope this helps!