Q. How do I cite a glossary from an on-line association such as National Association for the Gifted Child?

This is a bit tricky to answer without seeing the site. Also, APA does not have a specific example for an item like this. We will have to try and follow the spirit of the law. Since a glossary is generally a reference tool, we could base the citation on an entry for a reference work from p. 18 of the APA Style Guide to Electronic Resources, 6th edition.

23. Entry in a reference work, no author or editor

Major depressive disorder. (n.d.). In Merriam-Webster’s online dictionary (11th ed.). Retrieved from http://www.merriam-webster.com/medical/major+depressive +disorder

Based on the information provided in your question, an entry might look like this -

Gifted child. (2015). In National Association for the Gifted Child [Online glossary]. Retrieved from nagc.org/glossary