What clasps are used for closing lockets?
I know what you mean about terminology being the key. My quick pop of research shows me that you might want to use "friction clasp" as the search terms, especially as a number of sites which return from this have direct reference to locket construction.

The publisher of this page attempted a friction clasp closure for the locket and was not happy/successful in the result and created an alternative, which may also suit your purposes.



The selected solution is shown below:

 

One can see the pin was removed from the first attempt and the hole was not filled in. I believe one can still refer to the solution as a friction clasp, from a mechanical standpoint.

Another site goes into great detail regarding making a locket including the creation of a friction clasp, which appears some distance down the page. It appears that the process results in the rejected design of the first page I've listed, yet I suppose it's personal taste and ability split down the middle: