What is Back-end ? Only UI or also Devices / appliances

Question

I had this discussion with a friend.

So what is backend ?

Obviously User-Interface is front-end

In his opnion devices like heart-rate monitor is als front-end because they generate input.
In my opnion devices are back-end because the user does not touch it.

What is your opnion ?

Answer 1

Back = those codes work in back of the project
front = these codes work at the front of client