A teacher
An authority on knowledge — sometimes welcome, sometimes outgrown.
A teacher in a dream is the dictionary's image of an authority on knowledge. The reading depends on whether the teacher in the dream is welcome or not. A welcomed teacher is the dictionary's image of guidance you have been ready to receive; an unwelcome one is the dictionary's image of an old standard you have outgrown but still feel measured against.
Where you often see it. A teacher from your past appearing in your present. A teacher whose face is generic but whose authority is precise. The dream uses the teacher for an authority on knowledge — sometimes welcome, sometimes outgrown.
What it is not. The dream's teacher is not always to be obeyed. The reading may be about a knowledge structure you have grown past.
Related in the dictionary. Read teacher with school, book, writing, and any scene where you are being asked to learn something the dream is already half-naming.