Your CPU is the central processing unit, or the main body of your computer. A peripheral unit is anything attached to it.

No, the printer has a internal processor that manages its functions, but yes, it is a device that connects to the system unit.

the computer's processor HELPS managing it, but the printer's processor do the hard stuff

Yes, peripheral is a noun for a device or unit separate from but connected to a computer. The word peripheral is also an adjective, a word that describes a noun.

magnesia

Milk of magnesia is an aqueous suspension of magnesium hydroxide, Mg(OH)2. Each formula unit of magnesium hydroxide contains 5 atoms (one magnesium, two oxygen, two hydrogen).