An indicator is either an identifier, a mode indication or an operator. In all cases it specifies or denotes some other entity: identifiers specify the internal objects ascribed to them in identity declarations, mode indications specify modes associated to them in mode declarations, and operators specify routines ascribed to them in operation declarations.
The indicator in the following identity declaration is pi:
real pi = 3.14;
The indicator in the following mode declaration is tree_node:
mode tree_node = struct (int payload, ref tree_node next);
The indicator in the following operation declaration is +:
op + = (tree_node n1, tree_node n2) tree_node: ...;
[RR 4.8.1.A,G]:
A) INDICATOR :: identifier ; mode indication ; operator. G) TAX :: TAG ; TAB ; TAD ; TAM.