On Wikipedia, it is defined as "Sensei (can be pronounced "Sensai" as well), Sinsang, Sonsaeng, Seonsaeng or Xiansheng (先生) is an honorific term shared in Chinese honorifics and Japanese honorifics that is translated as "person born before another" or "one who comes before". In general usage, it is used, with proper form, after a person's name, and means "teacher"; the word is also used as a title to refer to or address other professionals or persons of authority, such as clergy, accountants, lawyers, physicians, and politicians  or to show respect to someone who has achieved a certain level of mastery in an art form or some other skill, e.g., accomplished novelists, musicians, artists and martial artists."
Websters Definition of sensei: "a teacher or instructor usually of Japanese martial arts (such as karate or judo)"
The meaning is so much more for a karate teacher, especially when teaching kids karate. The first definition for kids in karate is that of a teacher like in school. Their definition should grow over time to encompass a greater feeling of compassion. We nurture this by talking about the newer kids in karate class. The idea is that we are all on the same road some much further some only steps away. We can see the pat of newer student because we were recently there. Its more like follow me and by the way here is how to avoid that pothole ahead of you.
When we all start our day or karate class with this concept we ad more value to the world and each other. Our society has changed so much that mentoring has fallen off. I use to visit my little league's coaches house, or my father's friends who I looked up to, they would give me advise or correct my behavior. I was able to hear them more so than my parents. Today we are not going to have our kids roam free like in times gone past, but you can get similar benefits by placing them in activities Like Kids Karate.