Difference between aikido and karate?

As it applies to the physical aspect of self defense, they each use a different approach to dealing with an attack.

Aikido = mostly avoidance and redirection/parry to joint manipulation, throws and projections.

Karate = mostly forceful blocks, and damaging strikes with hands and feet.

In general terms of fight analysis, an opponent may attack by striking, throwing, or grappling techniques. The defender may likewise strike (or counter-strike), throw, and/or grapple in any order or combination. As for defensive tactics against a strike, the defender can either avoid being hit, block with a counter-strike to the attacking limb (typically at a 90 degree angle and with significant force to abruptly stop and possibly damage the limb), or redirect via parry.

"Karate" is a term that has been used to describe many different interpretations of Asian Martial Art training. Originally, it was a term applied by the inhabitants of the island of Okinawa. After learning of the Chinese hand fighting (called "te" for "hand" in Japanese), the Martial Artists of Okinawa modified the curriculum to create Okinawa-te. To distinguish this from the earlyhand fighting from the T'ang Dynasty, the called the Chinese system "Kara-te" (Karla-tay), meaning "ancient hand,""T'ang hand" or "China hand." Later, this system spread from Okinawa throughout Japan, and became "Karate-do" (the way of the "Empty Hand") with a change in the Chinese writing as suggested by Shotokan founder, Gichen Funakoshi. Since that time, "Karate" has also become a generic term to mean any unarmed combat or self defense in general, and can refer to any system under any other name or country's origin.

Typically, Karate is known as a stand-up method of sparring with an opponent by avoiding attacks, blocking with force, and counter-striking with mostly closed and open hand strikes, elbows, knees, and some essential kicks. The objective is to damage the opponent with impact force from strikes and cause debilitating injury, knockout, or death as needed to thwart an attack and defend one's self from eminent harm.

Aikido was named as the way of "harmony" by founder Morihei Ueshiba to describe the concept of avoidance and blending of mind, spirit, and physical energy with your attacker - to "join forces" rather than resist your attacker's energy. The approach to dealing with a strike is to either evade, or gently, but firmly intercede and redirect with a parry - often with the open blade of the hand. The attacking force is assisted and either propelled in a linear direction, reversed back upon the attacker, or takenfluently in a circular direction while maintaining a constant state of off-balance of the opponent.

In Aikido, the attacker is typically sent stumbling or falling, or is brought down and under control through the use of joint locks or joint manipulation that tend to hyper-extend joints and muscles, compress joints, or twist limbs to cause pain and discomfort that will bring about submission, cooperation, or accelerate the attacker into various throws or projections.