[Jaygles]

In my opinion, a very generic question like that deserves a very generic answer, with a follow-up asking if that is what they had in mind.

"An interface is roughly how a system is designed to be interacted with. A web page can be an interface with your bank if they have online banking. An API can be an interface for a back-end service to provide to other back-end services. Did you have anything specific in mind?"

[ponector]

Usually the interviewer has particular definition in mind and you need to guess it to pass to the next stages.

[limininsklyy]

Yup. I have been rejected once because I discussed a variety of options to harden a system instead of blurting out "VPN" as they wanted. They told me that after I discussed some trade offs in security for a little bit.

[Aeolun]

Anyone asking that question is generally ok with any of the answers. The important thing here is the concept, not the implementation.