Moment of contact, establishing the connection, connection point (literally "hitting the target")
Endō Seishirō sensei made the concept of atari – which is not so common in aikidō as in other martial arts – a focus of his practice.
There is a stickiness that tori must strive for in order to feel the ki of uke. The connecting point where uke and tori meet is the atari point. Only by maintaining this point can center and ki be transmitted to one another.
This is not to say that this point is fixed or static. It can move up and down your body, it can be maintained with the hands, arms, shoulders, hips, even the head. This connection point shifts and moves during practice and it keeps the two as one unified body.
This contact can only be achieved with completely relaxed shoulders and arms, but with a strong centre. The strength of your centre must radiate through your relaxed upper body. It is particularly important for a good connection that the wrists remain relaxed.
(Modified quote from The Dragon's Orb)