Using numpad in Vi (Vim) via PuTTY

Question

The numberpad does not work properly when using Vim through PuTTY. Instead of numbers I get

y
x
w
v
u
t
s
r
q

Answer 1

The previous answers by kirby, michael-berkowski, and pykler are great.

I had this issue in Vim on macOS's terminal. Checking Terminal → Preferences → Profiles → Keyboard showed that "Clear (Keypad)" was mapped to "Num Lock". Pressing clear fixed the issue for me.