A key on a Mac identified by a "square with a circle adjoined at each corner" symbol. It looks like a four leaf clover to some.

Some keyboards also include an Apple icon on the key, which has lead to many computer lab teachers mistakenly refering to the key as the 'apple key'.

The command key is used as a modifier key, in conjunction with other keys, to issue commands to the computer, like Command-O to open a file, and Command-C to copy selected text. It works similarly to the Control key on Windows machines.