Chief is a noun meaning the leader or ruler of a people or clan.
In modern usage, chief can refer to:
1. The highest-ranking officer in an organization (e.g. Chief Executive Officer)
2. The most important or main part of something (e.g. chief reason)
3. In American slang, used as a friendly term of address (e.g. "Hey chief!")
4. In cooking, the head chef (e.g. chief cook)