See Section 13.6.2 for points in the plane.
With the 3-d geometry screen in point mode, clicking on a point with the left mouse button will choose that point. Points chosen this way are automatically named, first with A, then B, etc.
Alternatively, the point command chooses a point.
Many commands which takes points as arguments can either take them as point(a,b,c) or the list of coordinates [a,b,c].
Example.
Input:
or:
Output:
(The marker used to indicate the point can be changed; see Section 13.3.2.)