(δοῦλος/doulos

This word is a type of person. It is a noun.

This word can mean slave. A slave is someone who is owned by another person. They are forced to serve that person.
This word can mean servant. A servant is someone who serves another person. They are dedicated to one person. However, they are not forced to serve this person.
This word can mean bondservant. A bondservant is someone who served another person because they needed to pay a debt. That is, they served someone because they owed money to this person.