One translation is: The way of combining energy.
You can use many words to replace energy but it is the Ki in Aikido. So many people ask just what is Ki? You may get many different answers, depending on who you talk to and from what culture they stem but here's a definition I have come to understand.
Consider life in any living creature.
In humans you might think of is as the breath of life or even the life spirit. It is the energy or power that flows through our bodies that makes us living creatures. Think of the breath in your lungs, the blood that pumps through your body as the heart beats. The electric burst of power that happens when the synapsis in your brain fire billions of times per second. This is all part of the Ki energy that flows through us all. Where does it come from? Another important question that you may find many different responses to. Some say God, others say the earth or the wind or the universe. No matter where it originates we all live thanks to the Ki energy in our bodies.
Ki, as I know it from the Japanese martial art of Aikido and the Korean martial art of Hapkido is also known in many different cultures as Qi, Chi, Gi, Ghi, X, and many more. So many differing ways to spell the same thing, yet they all recognize it as being some kind of energy.
Now let's look at the role of Ki energy in martial arts and in Aikido specifically. As Akidoka, we learn to channel this energy into our techniques by focusing our power into our center. This is where the human body is the strongest. Our center of gravity can be found in the middle of our bodies, to be more specific it is the a place in our abdomen just above the groin and below the navel. When performing a technique such as kote-gaeshi or a wrist lock, you focus your power in your center when you turn uke's wrist.
Further more in 99% of all Aikido techniques you will focus your power here in your center. This makes you stronger than your opponent in most cases or allows a much weaker opponent to break the balance of a stronger one.
Consider this, when a person tries to hold you by grabbing your wrist with both hands they can pull you and control you if you are off balance. Yet, if you find your center of gravity and use the strength of your body and not just the pulling motion of the one grabbed arm you can break away from the hold or break the balance of the attacker and then use a technique to breakaway, pin or throw.
There are some schools that take the spiritual approach to defining Ki and even some who try to teach it as a supernatural force with mystical origins. This is fine and I say to each his own but remember we all have Ki but it takes training and practice to focus that energy and use it properly when it comes to martial arts.