My textbook defines a reference frame in two different ways:
A collection of at least 3 collinear points that are rigidly connected.
A reference frame is defined by three orthogonal unit vectors and one point (the origin).
These two seem like two completely different definitions to me. Could someone explain what the connection is between these two definitions? The textbook attempts to explain this by writing "a reference frame is equivalent to a rigid body. Since all points of a rigid body are fixed with respect to one another, we can use them to define a reference frame", but I am still having trouble understanding the connection.