cod liver oil?

Hello,

I've read this sentence, from Stephen King's It (about a shelf with vitamins:

"There's iron, calcium, and cod liver oil."

What does "cod" mean here?

I've taken a look here:
http://www.collinsdictionary.com/dictionary/english/cod?showCookiePolicy=true

but neither of them seem to make sense (at least to me) when associated with the "liver oil".

Can anyone please clarify this to me?

Thanks!