# Terms The [Language overview](overview.ipynb) introduces these terms informally, as they come up. Every [value](#term-value) is an [atom](#term-atom) or an [array](#term-array): atoms are [numbers](#term-number), [characters](#term-character) and [functions](#term-function), [scalars](#term-scalar) are arrays of [rank](#term-rank) 0, [units](#term-unit) are atoms and scalars, and [subjects](#term-subject) are all values except functions.

Term	Meaning
value	a number, character, function or array
atom	a value that isn’t an array: a number, character or function
number	an approximate real or complex number, or an exact integer or rational, as Numbers describes
character	one Unicode character, written in single quotes: `'a'`
function	a primitive, dfn, train or derived function, which applies to arguments
pervasive function	a function that applies to each atom of its arguments, through any nesting: `1+[2 ⊂3]` is `[3 ⊂4]`
array	a rectangular collection of values along zero or more axes
rank	the number of axes: a vector has rank 1, and atoms and scalars have rank 0
scalar	an array of rank 0, holding one value: `⊂5` is a scalar holding the number 5, and `5` is a number
unit	a rank-0 value: a number, character, function or scalar
subject	a value that isn’t a function: a number, character or array
cell	a sub-array made of an array’s last k axes, called a k-cell: a matrix’s rows are its 1-cells. A 0-cell is the unit at one position, the atom there or the array there enclosed. When k is at least the array’s rank, the whole array is its one cell
frame	the axes before the cells, with one cell at each position
assembly	building one array from results, one for each position of a frame: the shape is the frame followed by the results’ shape, each result fills its position with its items, and shorter results are padded with fill
literal	a number, character, string, `∞`, `⍬`, or a constant: `$t` true, `$f` false or `$n` NaN
strand	subjects side by side, read as a vector without brackets: `1 2 3` is `[1 2 3]`, and `a b` is `[a b]`
run	a sequence of tokens with no spaces between them
argument	a subject a function applies to
operand	a value an operator applies to
expression	a run, group or statement that ends in a subject
train	a run, group or statement that ends in a function
fork	three parts of a train, `f g h`, giving `(f ⍵) g (h ⍵)`, or `A g h` with a subject on the left, giving `A g (h ⍵)`
atop	two parts of a train, `f g`, giving `f (g ⍵)`
bind	a subject directly before a function in a train, fixing its left argument: `2×` is `2↣×`