Mac OS X is shipped with a built-in speech recognition feature called Dictation, which comes in handy when you want to write documents, emails, or fill up online forms. Dictation allows you to speak out your text instead of typing it, leaving your hands free to perform other tasks.
How To Enable OS X Enhanced Dictation
The Dictation feature requires a permanent internet connection, as voice recognition is server-based (processed on Apple's servers). But if you would like to use Dictation without an internet connection, you simply have to enable Enhanced Dictation (CPU-based voice recognition).
Click on the Apple menu > System Preferences > Dictation & Speech > Dictation and then tick the Use Enhanced Dictation checkbox. Mac OS X will then download the files required to run Dictation offline.