In Japan anyone who teaches anything is called sensei. A young kindergarten teacher of 20 years old can be addressed as sensei by a parent of 50 years old because the parent appreciates the fact that the young teacher is teaching the child. This is similar to using the Mr, Miss or Ms - it merely indicates respect toward the person. In Japan a lawyer or a doctor is normally called sensei because they are supposed to know more than others. Since lawyers often become politicians, even politicians are often called sensei. It is true that in Japan the word sensei is sometimes abused. In general however, a Japanese teacher is usually addressed as sensei by his students all over the world.
There is also the question of what makes one a sensei. When one teaches, one is a sensei. But one might not be a sensei if one is not teaching. It depends on what one teaches. If one teaches Aikido techniques, one is a teacher only on the tatami. If one teaches only on the tatami for a few hours, it is inconvenient to keep changing one's title on and off the tatami. People who are called sensei are supposed to behave as a teacher in daily life as well. In this way people can continue to refer to them as sensei without the need for a change of titles.
This creates a new question: How should a teacher behave in his daily life? Zen Buddhism began the idea of understanding life. It suggests that when one understands life, one feels as if one is newly born. Sen means before and Sei means to be born. So sensei literally means a person who is born before. This birth is a spiritual, not physical birth. The original meaning of sensei is one who is spiritually born before others. If one sticks to the original meaning, only those who understand life should be called sensei and a sensei is behaving like a teacher 24 hours a day, even in his daily life.
Respect used to be very important to all people all over the world. One should respect human beings, animals, trees, etc. In other words, only life and living things should be respected. One should not respect materials. Even in the case of highly technological machines like Ferrari cars or great works of art, it is not the object that should be respected but the human beings behind the creation of the object. One respects life because each life is related to other lives and these millions and billions of relationships create and maintain life on Earth. Respect creates better relationships between lives and that is why respect was and is one of the most important things in life. Respect actually means not disturbing the relationships among different lives.
When respect was demonstrated, it was called honour. People used to be ready to die for honour because the relationships among lives were essential for survival of human beings. These days both respect and honour are almost forgotten because we have developed a civilization based on weapon and material. Survival depends on weapon and politics and not relationships among different lives anymore.
Aikido is a way (Do) and the way should be maintained in aikido practice. The person who keeps the way is called Doshu. There are two different meanings of doshu in Japanese. The first means 'master of the way' and the second means 'keeper of the way'. The difference is that there can be only one master of the way but many keepers of the way. The top teacher of any aikido organization should be called doshu - keeper of the way - but actually many aikido organizations do not have doshu. This is because most Japanese teachers actually follow the teachings of somebody else, especially Morihei Ueshiba, even though he is dead. There are also teachers who do not follow a particular teacher but instead follow some established philosophy. They too are not keepers of the way. That is why there are not many doshu in the aikido world. A teacher is doshu only when he teaches completely independently of any philosophies or other teachers including the dead ones.