arm - What does 'bank'ing a register mean?

Question

Reading 'ARM Architecture' on Wikipedia and found the following statement:

Registers R0-R7 are the same across all CPU modes; they are never banked.

R13 and R14 are banked across all privileged CPU modes except system mode.

What does banking a register mean?

Answer 1

Register banking refers to providing multiple copies of a register at the same address.

Taken from section 1.4.6 of the arm docs

The term is referring to a solution for the problem that not all registers can be seen at once.

There is a different register bank for each processor mode. The banked registers give rapid context switching for dealing with processor exceptions and privileged operations.

If your looking for a more theoretical reasoning, I recommend this paper.
Edit: A much deeper answer than mine is given here