A person who teaches or instructs (e.g., at a school, dojo); a teacher or instructor.
私の日本語の先生は毎回詳しく文法を説明してくれます。
My Japanese teacher explains grammar in detail every time.
小学校の先生は子どもたちの世話もします。
An elementary school teacher also takes care of the children.
ピアノの先生に週に一度レッスンを受けています。
I take piano lessons from a teacher once a week.
剣道の先生は礼儀をとても大切に教える。
The kendo instructor places a lot of importance on teaching etiquette.
An honorific title or form of address used to show respect (e.g., for teachers, masters, doctors, lawyers); often used as a suffix (〜sensei).
山田先生、疑問点があるのですがよろしいですか？
Professor Yamada, I have a question — is now a good time?
具合が悪いので、主治医の先生に診てもらった。
I felt unwell, so I saw my doctor.
税理士の先生に確定申告をお願いした。
I asked the tax accountant (先生) to handle my tax return.
A familiar or teasing form of address used jokingly among friends (calling someone 'sensei' ironically or affectionately).
彼は何でも知ってるから、みんなから『先生』って呼ばれているよ。
He seems to know everything, so everyone jokingly calls him 'sensei'.
料理が上手な友達に冗談で『料理の先生』と言った。
I jokingly called my friend who cooks well 'the cooking teacher'.
An archaic meaning: one’s elder or senior. This original usage is largely obsolete in modern Japanese.
古文には『先生』が年長者を意味する例がある（現代語では稀）。
In classical texts, 'sensei' sometimes means an elder (rare in modern usage).