What does 先生 Sensei mean?

Sensei is usually translated as teacher, and it is a title for anyone further along a path than the person speaking. Teachers qualify. So do doctors, lawyers, dentists, politicians, novelists and manga artists.

The characters

先 (sen) means before, ahead, previous. 生 (sei) means birth or life. Born before. It says nothing about instruction and everything about sequence, which is why the English translation keeps slipping.

Compare senpai, built from the same 先: someone ahead of you in a shared institution. Sensei is the same idea generalised past institutions to a whole field.

Why a doctor is a sensei

Because the title tracks expertise arrived at before you, not a job that involves teaching. A patient calling their doctor sensei is not confused about the doctor's profession. They are marking a relationship in which one party knows the terrain.

The manga case is the clearest. A manga artist is called sensei by editors, assistants and readers, and teaches nobody. What they have is a body of work and standing in a craft, which is sufficient.

The rule that trips people up

You never apply it to yourself. Japanese titles are relational: they describe where the other person stands relative to you, so claiming one is a category error rather than merely immodest. Introducing yourself as sensei sounds roughly like introducing yourself as Your Honour.

It can also be used with a knife in it. Applied to someone whose expertise is doubtful, or laid on thickly in the wrong setting, it reads as mockery. Politicians are called sensei routinely, and it is not always warm.

Why English keeps it untouched

Because the alternatives all import the wrong thing. Teacher narrows it to a job. Master drags in martial arts films and a servility that is not there. Doctor and professor are specific credentials, where sensei is deliberately unspecific about which path you are ahead on.

The gap in English is not conceptual. It is that English distributes this work across a dozen titles that each carry a profession, and Japanese has one that carries only the direction of travel.