"Sensei" (pronounced: Sen-say) is the Japanese word for "Teacher". That is all it means. It does not mean "Master" or anything of that nature.
Ki Aikido is teaching for our daily life - it is not just a matter of "moves" or techniques. So we would be foolish to trust this to someone we did not respect. Addressing our teacher as "Sensei" simply maintains that sense of respect. Most students consider it a privilege to be able to call their teacher "Sensei".
The normal Japanese use of the word "Sensei" is to place it after the surname, e.g. Abbe Sensei. However, as an English speaker, Sensei Williams prefers to use it before the surname in the same way we would use "Mr" or "Mrs".