a teacher: one who instructs others in what they should know and do
A teacher is someone who passes on knowledge and shapes how others live by it. In the Gospels the word clusters overwhelmingly around Jesus, where it is the plain Greek word standing behind the Hebrew title Rabbi, the respectful name a student gives the master he follows and learns from. Two Gospel writers even pause to translate Rabbi for their readers with this very word.
Beyond that title, the same word names a recognized role. In the early church a teacher was one of the gifts God gave the community, listed alongside apostles and prophets, and the role carried real weight: anyone taking it on would be judged more strictly for what they handed on. The word also works at the everyday level of any teacher and the student who learns from him, which Jesus turned into a proverb: a student is not above his teacher.
The senses are not far apart. They share a single picture, the person others look to in order to learn, and they differ only in who is in view: Jesus as the Teacher above all, the appointed teacher in church or in Israel, or a teacher in the ordinary sense. So one English word, teacher, fits them all; the shading comes from context, not from the word itself.