An "indication" is a mark or symbol of some kind. For an angle, that mark is often an arc (or double arc) near the vertex. The indication may also include a letter or symbol that can be used to refer to the angle. In the picture here, the indication (or identification) includes the Greek letter theta. (This picture is also showing definitions for arc length s and radius r.)
The angle on a diagram that has the only indication mark or identifier, or that has a particular mark, is called "the indicated angle." In this diagram we can refer to "the angle indicated by theta."