Is there a non-medical name for the curve where index finger and thumb meet?

I'm trying to write a short post about hand spinners, and one of the problems I have with a hand spinner is that it hits the curve between the index finger and thumb. I can't seem to find any good diagrams with terms for parts of the hand which aren't entirely populated by medical terms, so currently my options are "curve where index finger and thumb meet" and "adductor pollicis" to describe this part of the hand.

Is there a layman's term for that part of the hand?

I've heard it called the crook of your thumb.

An example of me seeing it used this way can be seen in the comic below.

Perhaps you are referring to the web of the thumb.

This sense of the word "web" is defined by Merriam-Webster as

a tissue or membrane of an animal or plant; especially : that uniting fingers or toes either at their bases (as in humans) or for a greater part of their length (as in many waterbirds)

Single-word answer: purlicue.

The adductor policis that you mention in your original question is actually the name of a muscle in that area. According to the Collins dictionary, the word you're after is purlicue:

(ˈpɜːlɪˌkjuː) the distance between one's forefinger and thumb when extended

Etymologically this is of unclear origin, but it appears to be chiefly British (as seen in the other definitions provided by Collins), and probably derives from Scots word pirlie, which means curly, as per the shape of the hand between the thumb and forefinger.