A definition is a list of named items (which may be numbers, strings, lists,...).
definition definition(any,...)
Builds a definition with the specified elements. Note that the keyword definition is not in fact required:
my_field = (param : "z", level : 500, grid : [2, 2])
any definition[string]
Returns the value named by the input string from a definition.
level = my_field["level"]
Note the shorthand form of this, the dot operator:
level = my_field.level