AI KI DO is compound word made up of 3 kanji each of which can mean many different things depending on the context in which they are used.
The word Aikido did not exist before the 2nd world war but was coined about 1942 or 43 in an attempt to explain the martial art that Morihei Ueshiba was in the process of developing.
AI: these days it is commonly translated as Love, but Love is written with an entirely different kanji although it is pronounced the same. Ai in the context of Aikido more than likely means meeting, coming together, joining together, or fitting together. Coming together or joining together is exactly what happens when Uke and Nage combine to create an aikido waza. It does not necessarily mean harmony which is what most people like to think it means.
AIKI is a word which describes the concept of energy flow within an individual and the use or direction and control of that energy to defeat an opponent. Many other martial arts exhibit the use of AIKI so it is not only apparent in Aikido. (Most notably Aiki-Jutsu, a precurser to Aikido and from which Aikido has evolved many technical applications.)
KI of course means spirit, energy, universal energy that can be found in everyone and everywhere, and this kanji is used in many different ways to describe different kinds of energy such as electricity, steam vapour, feelings, personal energy, and is a concept difficult to describe and quantify.
DO means the way of understanding or studying something, a path that is followed towards the understanding of something, a road, and other similar things including both philosophical as well as practical approaches, again depending on context. It is more embracing overall than Jutsu which refers to practical techniques for battlefield survival.
Aikido is a difficult concept to explain in words because it can be interpreted in many ways. The only way to understand it is to actually practice it with a partner in the dojo.
The simplest definition for those beginning to study Aikido is to say it is: A way of, or study of Harmony through the use of Ki energy or spirit.