Real Dictionary

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

senior in American English
of the greater age; older
adjective: older or elder (usually designating the older of two men bearing the same name, as a father whose son is named after him, often written as Sr. or sr. following the name)
noun: a person who is older than another