The full name is primary expression. The meat of it is that a primary is usually just a literal value, identifier, or something in parentheses, but it varies based on the particular language.

For a deep dive you'll want to read resources about parsing. To name one article that came up recently on HN, take a look at "Why Parsing Tools are Hard" [0].

[0]: http://blog.reverberate.org/2013/09/ll-and-lr-in-context-why...