If someone is senior to you in an organization or profession, they have a higher and more important job than you or they are considered to be superior to you because they have worked there for longer and have more experience.
Senior is used when indicating how much older one person is than another. For example, if someone is ten years your senior, they are ten years older than you.
Seniors are students in a high school, university, or college who are the oldest and who have reached an advanced level in their studies.
ADJ
(比赛)成人的，高水平的
If you take part in a sport at senior level, you take part in competitions with adults and people who have reached a high degree of achievement in that sport.
In sports such as golf and tennis, a senior is a professional player who is fairly old and who plays in special competitions against other older players.