Someone who is much senior to you in age or experience; a senior veteran who commands respect (often used in workplaces or industries).
彼はこの業界の大先輩で、会うといつも貴重なアドバイスをくれる。
He is a great senior in this industry and always gives valuable advice when we meet.
大先輩の話を聞いて、仕事に対する姿勢を改めた。
Listening to the senior's story made me change my attitude toward work.
失礼がないように、大先輩には特に丁寧に接したほうがいい。
To avoid rudeness, it's better to be especially polite to someone greatly senior.