The differentiated relative movement of separate elements, allowing the expression and communication of information.
The articulated lorry; the articulation of the knee; the lazy speaker's technical problems with articulating the words; the well-spoken, articulate politician who can communicate his ideas - these various meanings are much more closely related than one might expect.
The primary articulation of psychology is between the eyes and the face. We see this in the modern socialized human, and in naturalistic (especially film) acting. The primary articulation of biology is between the head and the body. We recognize this in the animal kingdom and the use of the theatrical mask.
To illustrate, if a human entered a room she might stop, freeze, and scan the room with just her eyes. If a dog entered a room and stopped, it would use its whole head to look around. If a dog actually were to survey a room by moving just its eyes, it would appear to be possessed by an evil spirit; a sorry comment on our own species!