The senior people in an organization or profession have the highest and most important jobs.

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.

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.

Your seniors are the people who are senior to you in an organization or profession.

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.