Uses undefined terms

In 1.6, Create Test Accounts, in section Step 2, the following text appears:

> Notice that actually alice has both owner and active public keys. EOSIO has a unique authorization structure that has added security for your account. You can minimize the exposure of your account by keeping the owner key **cold**, while using the key associated with your active permission.

This is the first appearance of the term **cold**. No explanation of that term is provided. So, the reader is left trying to figure out what cold means in this context.