From J Wiki
Absolutely Essential Terms
To make headway with J, even to read NuVoc with profit, you need a clear idea of what these terms mean.
For a more extensive list of J terminology and other helpful terms, see: Vocabulary/Glossary.
TERMINOLOGY EXPLANATION Sentence An entire executable line. A sentence may not span lines. More... Word A sequence of characters in a sentence, recognized as a lexical unit. A word is either a name, a primitive, or a constant. More... Verb A "function". In a sentence, a verb operates on the noun to its right (and its left, if a noun is to its left) to produce a noun result. More... Noun An atom or array of atoms. Nouns hold the data that verbs operate on. More... Atom Any single number, single character, or single box. Also called a scalar. An atom is a noun with rank 0 . More... Array A noun comprising atoms arranged along one or more axes. J arrays are rectangular. More... [Array] Item A subarray whose rank is one less than the array's. An atom has one item: itself. The items of a list are atoms; the items of a table are lists. More... Box Any noun can be enclosed in a box to create a new noun which is treated as an atom. Boxes can be arranged in an array even if their contents differ in shape or type, to produce a heterogeneous array. More... Shape The list of the lengths of the axes of a noun. More... [Noun] Rank The number of axes along which the atoms of the noun are arranged.
[Verb] Rank The highest rank(s) of the noun argument(s) that the verb can operate on. More... List An array of rank 1. A one-dimensional array. A string or vector. Table An array of rank 2. A two-dimensional array. A matrix. Locale A public namespace. Each public name is defined in a locale. More... Argument A noun that a verb works upon. There is always a noun to the right (the y-argument). There is sometimes a noun to the left (the x-argument). Operand A noun or verb that a modifier works upon. Not the same as an argument.