The following are some definitions I have come up with from the aikido literature (I'm only reporting so those of you that detest the topic please give me a break): set up blows for techniques; strikes to vital points for distraction or unbalancing; blows to disturb ki; control and balance taking (kuzushi) blows; termination of the attacker; a neutralizing force; a blow to end confrontation causing disability, unconsciousness, or death; a method to facilitate technique due to pain compliance, energy shifting or posture alteration; and use as a method to control the attacker's mind.
Atemi can be used to distract the attacker, break balance, set up technique, cause coordination difficulties, shut down or alter the nervous system, cause numbness or weakness to limbs, damage tissue, and kill.
Disclaimer: These definitions and comments are from the aikido literature and an internet survey. They are not the end all of possible definitions and some aspects may not fit the aikido paradigm of some aikidoka. Multiple influences affect how one perceives the topic: prior training and experience; philosophy; legal issues; one's paradigm of aikido; organization membership; and interest.