In Sierra, the default "keyword phrase word for dictation commands is "computer".

Experience has taught me a lot about choosing dictation command keyword phrases...
  • do NOT choose the "Listen Without Requiring Keyword Phrase" option
  • It is tempting to use Siri as the keyword phrase but Siri is a specific functionality and that can cause confusion
  • do not choose a keyword phrase that is apt to come up in common conversation.
  • the best keyword phrase is short, very distinct, memorable and very obscure
  • turn off dictation commands if you are watching a moving or there is a lot of background conversation.
  • if you have dictation commands enabled and apps or windows start opening or closing mysteriously, the cursor starts wandering all over the screen, or mysterious text snippets start appearing in the document you are working on — turn off dictation commands.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein