A primitive function is a function defined by the language. Outside of the array community, such functions may be called “builtin” or “intrinsic” functions. In APL, each is represented with a single glyph; in other languages, such as those restricted to ASCII characters, they may use multiple characters (“bigraphs” and “trigraphs” are combinations of two and three characters, respectively). Other parts of APL which are written with a single glyph include primitive operators and Quad.
A function is distinct from the glyph used to denote it. Different APLs, or even one APL (using migration level) might use the same glyph for multiple functions, or different glyphs for identical or similar functions. The term “function” can, depending on context, refer either to an ambivalent function which can be applied with one or two arguments, or the monadic or dyadic function obtained by restricting that function to either one or two arguments specifically.