Aikido is a Japanese budō martial art. Aikido is also a philosophy. The practice of Aikido is said to be a lifelong pursuit that doesn’t end in the dojo. It goes on continuously through one’s life, fighting to achieve balance, peace and an enlightenment of the mind along with the spirit.
'Ai' is harmony in the sense of uniting and combining as one.
'Ki' translates into life-force. This principle is the same as the more commonly used Chinese term 'qi' (ch'i). Everyone has a 'ki', our 'ki' is forever changing, it's our inner life force.
'Do' is 'The Way'. You might have heard of this term from books or films about Japanese martial arts. It is often used in Zen to refer to not only the individual path one's life takes, but to also understand it and develop and awareness of the greater things around us.
In its simplest explanation Aikido is the merging of the above principles through the discipline of a martial art.
"The journey of a thousand miles begins with a single step."